A control and suspension system for a retractable covering mounted on a rotating element includes an apparatus for mechanically limiting over-extensions of the covering and an apparatus for mechanically limiting over-retractions of the covering. The control and suspension system also includes an apparatus to compensate for any undesirable skewing of the covering that might occur. Finally, the control and suspension system also includes a bottom rail that attaches to the bottom of the covering by trapping a portion of the covering between a compression plate and a bottom plate.

CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a division of U.S. Nonprovisional application Ser. No. 09/741,240, filed 18 Dec. 2000 (the '240 application), now U.S. Pat. No. 6,435,252 B2, issued 20 Aug. 2002, which is a division of U.S. Nonprovisional application Ser. No. 09/338,332, filed 22 Jun. 1999 (the '332 application), now U.S. Pat. No. 6,289,964, issued 18 Sep. 2001, which claimed priority to U.S. Provisional Application No. 60/090,278, filed 22 Jun. 1998 (the '278 application). The '240 and '332 applications and the '278 application are hereby incorporated by reference as though fully set forth herein. The '332 application also is related to U.S. Nonprovisional application Ser. No. 09/050,507, filed 30 Mar. 1998, now U.S. Pat. No. 6,116,325, issued 12 Sep. 2000 (the '325 patent), which claimed priority to U.S. Provisional Application No. 60/041,791, filed 2 Apr. 1997 (the '791 application). The '325 patent and the '791 application are both hereby incorporated by reference as though fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     The instant invention is directed toward a control and suspension system for a covering for architectural openings. More specifically, it relates to hardware for suspending and controlling the operation of a panel used to cover an architectural opening. 
     b. Background Art 
     It is well known to place coverings over architectural openings. It is also well known to make these coverings retractable so that the architectural opening may be exposed or hidden as desired. A common problem with the use of such retractable coverings is ensuring that the retractable covering is not over-extended or over-retracted. For example, if an architectural covering that is mounted on a roll bar is over-extended, it may detach from the roll bar. This type of detachment is highly undesirable and may damage the architectural covering permanently. If a window covering that is mounted on a roll bar is over-retracted, that is also highly undesirable. For example, if the covering is over-retracted, it may jam in the head rail, making the architectural covering unusable. Another common problem that occurs with retractable coverings is skewing of the covering as it is retracted. For example, if the architectural covering is mounted on a roll bar, it may wind onto the roll bar unevenly or unwind from the roll bar unevenly for a variety of reasons. Such uneven winding or unwinding is known as skewing. Skewing may result from a manufacturing defect, an error in hanging the retractable covering in proximity to the architectural opening, wear on the hardware and support system, or a variety of other reasons. 
     Various suspension and control systems have been proposed heretofore to address these common problems with retractable coverings for architectural openings. There remains, however, a need for more efficient means of compensating for the above types of problems encountered during the use of retractable coverings for architectural openings. 
     SUMMARY OF THE INVENTION 
     It is desirable to have a control and suspension system for retractable coverings or barriers that avoids over-extensions and over-retractions of the retractable covering. It is also desirable that the control system be able to compensate for any undesirable skewing that might occur. Accordingly, it is an object of the disclosed invention to provide an improved control and suspension system for retractable coverings. 
     A more detailed explanation of the invention is provided in the following description and claims, and is illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view in partial section of a retractable covering for an architectural opening in an extended configuration; 
     FIG. 2 is a left-end view of the retractable covering depicted in FIG. 1 with the covering in a fully retracted configuration; 
     FIG. 3A is a fragmentary sectional view taken about line  3 A— 3 A of FIG. 2, depicting control system hardware; 
     FIG. 3B is a fragmentary view of the covering depicted in FIG. 3A, depicting skew compensation; 
     FIG. 4 is a downward fragmentary cross-sectional view taken about line  4 — 4  of FIG. 2, depicting control system hardware; 
     FIGS. 5A,  5 B, and  5 C together depict an exploded isometric view of control system hardware located at each end of the head rail; 
     FIG. 6A is an isometric view of hardware also depicted in FIG. 5A, but from the opposite direction; 
     FIG. 6B is an isometric view of the releasable mounting plate, the other side of which is depicted in FIG. 5C; 
     FIG. 7 is a cross-sectional view of the clutch mechanism of the control system taken about line  7 — 7  of FIG. 4; 
     FIG. 8 is a cross-sectional view of the clutch mechanism of the control system taken about line  8 — 8  of FIG. 4; 
     FIG. 9 is a partial sectional view of the left end of the bottom rail taken about line  9 — 9  of FIG. 1; 
     FIG. 10 is a view of the inside surface of a bottom rail end cap, depicting the projections extending from the inside surface of the bottom rail end cap; 
     FIG. 11 is a top planform view of the bottom rail end cap depicted in FIG. 10; 
     FIG. 12 is an end view of the compression plate, which forms a portion of the bottom rail; 
     FIG. 13 is an end view of the bottom plate, which forms a portion of the bottom rail; 
     FIG. 14 is a fragmentary cross-sectional view of the bottom rail and a portion of the covering taken about line  14 — 14  of FIG. 9; 
     FIG. 15 is a fragmentary cross-sectional view of the bottom rail and the covering taken about line  15 — 15  of FIG. 9; 
     FIG. 16 is an exploded, fragmentary cross-sectional view of the bottom rail depicting how the first and second flexible sheets are attached to the bottom rail; 
     FIG. 17 depicts the control system hardware at the left end of the head rail, showing that the internal, roll bar support wheel moves left and right (as depicted) along the threaded shaft as the covering is extended or retracted; 
     FIG. 18 is an enlarged sectional view of a portion of the control system taken about line  18 — 18  of FIG. 17; 
     FIG. 19 is a second view of the control system depicted in FIG. 18, depicting abutment of the stopping ledge and the intercepting ledge; 
     FIG. 20 depicts adjustment of the control system hardware that controls the fully retracted configuration of the covering; 
     FIG. 21 is an enlarged cross-sectional view of control system hardware taken along line  21 — 21  of FIG. 20, depicting adjustment of the hardware that controls when during the covering-retraction process the covering is fully retracted; 
     FIG. 22 depicts the internal, roll-bar-support wheel installed in the roll bar, and shows the covering wrapped around the outer surface of the roll bar; 
     FIG. 23A shows the left end of the head rail in partial cross-section taken along line  23 A— 23 A of FIG. 4, depicting the covering approaching full extension; 
     FIG. 23B depicts the head rail components depicted in FIG. 23A, but shows the covering at full extension; 
     FIG. 24A depicts control system components shown in FIG. 23A in partial cross-section taken along line  24 A— 24 A of FIG. 4 as the covering approaches full extension; 
     FIG. 24B shows the control system hardware depicted in FIG. 24A after the covering has reached full extension; 
     FIG. 24C is a fragmentary cross-sectional view taken about line  24 C— 24 C of FIG. 24B; and 
     FIG. 25 depicts, in partial cross-section and partially broken out, control system components that facilitate skew adjustment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates most directly to devices  10  for covering architectural openings and control systems for retractable coverings or barriers for architectural openings. A sample of the type of covering contemplated for use with the disclosed control system is depicted in FIG.  1 . In this figure, the covering  12  comprises a first flexible sheet  14 , a second flexible sheet  16 , and substantially horizontal vanes  18  attached between the first and second sheets. A bottom rail  20  is attached to the first and second flexible sheets in a manner more fully discussed below. The upper end (as depicted) of the covering is attached to a roll bar, which is not visible in FIG.  1 . The control system hardware responsible for limiting the travel of the covering (i.e., the hardware that sets the fully extended position and the fully retracted position of the covering) is incorporated into the head rail  22 . The head rail  22  comprises a left end cap  24  and a right end cap  26 , and includes an arcuate cover plate  28 . The head rail  22  is attached to a support structure (e.g., a wall) by a pair of mounting brackets  30 . 
     FIG. 2 is an enlarged view of a portion of the left end of the apparatus  10  for covering an architectural opening. In this view an access door  32  through which the system components that control the fully retracted position is clearly visible. A slot  34  is formed into the left end cap  24 . In order to gain access to the control system hardware inside the head rail  22 , the access door  32  depicted in FIG. 2 is first removed by using a flat blade screwdriver, for example, into the door removal slot  34  molded into the left end cap  24  and prying the access door  32  from the door support ledge  44  (see FIG.  5 A). Once the desired adjustments have been made, the access door  32  may be popped or snapped back into position in the left end cap  24  to restore a more aesthetically pleasing appearance to the head rail  22 . Also, as depicted in FIG. 2, the covering  12  is fully retracted such that the bottom rail  20  is adjacent to the bottom side of the end caps  24 ,  26 . 
     FIGS. 3A,  3 B, and  4  depict fragmentary cross-sectional views of the head rail  22  taken along two perpendicular planes passing through the longitudinal axis of rotation of the roll bar  36 . In particular, FIGS. 3A and 3B show a partial cross-sectional view of the head rail  22  taken along line  3 A— 3 A of FIG.  2 . These views are taken along a vertical plane that passes through the longitudinal axis of rotation of the roll bar  36  incorporated in the head rail  22 . FIG. 4, on the other hand, is a fragmentary cross-sectional view taken along the plane containing line  4 — 4  of FIG. 2, which passes horizontally through the longitudinal axis of rotation of the roll bar  36  mounted in the head rail  22  depicted in FIG.  1 . The left end, as depicted, of these three figures show details concerning the skew adjustment features of the invention, and details concerning the system components that permit adjustment of an upper stop limit (i.e., the components that control how far the covering may be retracted). The right-hand end, as depicted in FIGS. 3A,  3 B, and  4 , show components of the control system that control retraction and extension of the covering via a clutch mechanism. The clutch mechanism used in the present invention is closely related to the clutch mechanism described in co-pending application Ser. No. 09/050,507, which has been incorporated herein by reference as though fully set forth in the present application. The reader should refer to this related application for details concerning the break away cord system used in the right-hand end of the head rail  22  of the present invention. 
     FIGS. 5A,  5 B, and  5 C together depict the major components of the control system  10  comprising part of the head rail  22  of the present invention. These three figures together comprise an exploded perspective view of components comprising the control system. Referring first to FIG.  5 A and the top half of FIG. 5B, the components associated with the left end, as depicted, of the head rail  22  are described first. Depicted at the left-hand edge of FIG. 5A is the access door  32 . The access door  32  covers the access port  42  in the left end cap  24 . When in position, the circumferential edge of the access door rides in a door support ledge  44  formed in the left end cap  24 . Also formed in the left end cap  24  is a slot  34  that permits someone desiring to make adjustments in the head rail components to remove the access door  32 . The access door  32  fits into position by pressing it into the access port  42  until it snaps or pops into position. 
     Moving from left to right in FIG. 5A following the dashed line, the next component encountered is the plunger  46 . The plunger  46  comprises a plunger head  48  followed by a large cylindrical portion  50 , an intermediate cylindrical portion  52 , a small cylindrical portion  54 , and two flexible arms  56 . A screwdriver slot  58  is formed into the plunger head  48 . The large cylindrical portion  50  has a cross-sectional diameter that accommodates a setting retention spring  60 , also depicted in FIG. 5A (see, e.g., FIGS. 3A,  3 B, and  4 ). The inside diameter of the generally cylindrical cavity within the setting retention spring  60  is slightly larger than the outside diameter of the large cylindrical portion  50  of the plunger  46 . As shown in FIG. 3A, for example, the setting retention spring  60  slides over the large cylindrical portion  50  of the plunger  46  when the head rail  22  is assembled. The diameter of the intermediate cylindrical portion  52  is slightly smaller than the diameter of a spring retention ring  62  (see, e.g., FIG. 3A) located inside a cylindrical housing  64  extending longitudinally from the inward side of a skew adjustment plate  66 . The spring retention ring  62  is an integral part of the skew adjustment plate  66 . In particular, the spring retention ring  62  is formed on the inner surface of the cylindrical housing  64  projecting from the skew adjustment plate  66 . In the assembled head rail  22 , the setting retention spring  60  is mounted around the large cylindrical portion  50  of the plunger  46  and is trapped between the underside of the plunger head  48  and the spring retention ring  62  of the cylindrical housing  64  that is part of the skew adjustment plate  66 . 
     As shown in FIG. 5A, the intermediate cylindrical portion  52  of the plunger  46  includes two interlocking channels  68 , which are offset from each other by approximately 180° in the preferred embodiment. As will be described further below, these interlocking channels receive interlocking tabs  70  of a threaded shaft  72  (see FIG.  5 B). Locking tabs  74  are located at the distal ends of the two flexible arms  56  of the plunger  46 . As explained in more detail below, these locking tabs  74  help ensure that the plunger  46  and the threaded shaft  72  in the assembled head rail  22  move as a single unit. 
     Continuing from left to right in FIG. 5A, the next components of interest are the skew adjustment plate  66  and a threaded skew adjustment plug  76 . The cooperation or relationship between the left end cap  24 , the threaded skew adjustment plug  76 , and the skew adjustment plate  66  is best seen by considering FIG. 5A in conjunction with FIG.  6 A and FIG.  3 B. As best seen in FIG. 6A, the left end cap has molded on its inner surface a plug bed  78 . The threaded skew adjustment plug  76  rides in the plug bed such that the screwdriver slot  58  in the bottom end of the skew adjustment plug  76  is accessible through an access hole  80 , which is also molded on the inner surface of the left end cap  24 . When the skew adjustment plate  66 , which also mounts the roll bar  36 , is positioned in a pair of the channels  82  located on the back side of the left end cap  24 , the threaded skew adjustment plug  76  is pinched between the bottom of the plug bed  78  (FIG. 6A) and an arcuate threaded surface  84  (FIG. 5A) on the left-hand side, as depicted, of the skew adjustment plate  66 . The skew adjustment plug  76  is thereby trapped in the plug bed  78  between the left end cap  24  and the skew adjustment plate  66 . The pressure exerted on the threaded skew adjustment plug  76  by the left end cap  24  and the skew adjustment plate  66  prevents the skew adjustment plug  76  from easily rotating, but it remains possible to rotate the skew adjustment plug  76  using a flat-blade screwdriver inserted through the access hole  80  molded in the left end plate  24  as depicted in FIG.  3 B. 
     Referring again to FIG. 5A, a roll-bar-end support wheel  86  and its associated down limit stop  88  are described next. As depicted, the down limit stop comprises three primary components: a mounting tang  90 , a wedge  92 , and an arcuate arm  94 . As depicted, the distal end of the mounting tang  90  is split, and a locking tab  96  is integrally formed on opposing sides of the mounting tang  90  adjacent to the split. The opposite end of the mounting tang  90  is integrally formed with one end of the arcuate arm  94 . The arcuate arm  94  includes an arcuate outer edge  98  and a substantially flat leading edge  100 . The wedge  92  is attached to the same side of the arcuate arm  94  as the mounting tang  90 , but the wedge  92  is attached adjacent, but not flush with, the leading edge  100  of the arcuate arm  94 , whereas the mounting tang  90  is integrally formed with the opposite end of the arcuate arm  94 . The wedge  92  includes an outer surface  102 , a leading edge  104 , and a trailing edge  106 . 
     The roll-bar-end support wheel  86  includes a mounting hole  108  that accommodates the mounting tang  90  of the down limit stop  88 . When the mounting tang  90  is properly inserted into the mounting hole  108 , the locking tabs  96  on the distal end of the mounting tang  90  rotatably lock the down limit stop  88  to the roll-bar-end support wheel  86 . Since the diameter of the mounting hole  108  substantially corresponds to the diameter of the mounting tang  90 , the locking tabs  96  snap outward once they pass an annular ledge  526  inside the mounting hole  108  (see FIG.  24 C). The portion of the mounting tang  90  between the back side of the arcuate arm  94  and the bottom of the slot existing in the distal end of the mounting tang  90  substantially corresponds to the length of the mounting hole  108  in the roll-bar-end support wheel  86 . When the down limit stop  88  is thus snapped into position onto the roll-bar-end support wheel  86 , and after the roll-bar-end support wheel  86  is positioned in the roll bar  36  (see FIG.  22 ), the wedge  92  of the down limit stop  88  rides in an elongated channel  110  (FIG. 5B) of the roll bar  36 . 
     The roll-bar-end support wheel  86  also includes an alignment groove  112 . The alignment groove  112  accommodates an alignment tongue  114  (FIG. 5B) comprising an integral part of the roll bar  36 . The alignment groove  112 , when slipped over the alignment tongue  114 , forces the roll-bar-end support wheel  86  to rotate in unison with the roll bar  36 . Also visible in FIG. 5A on the roll-bar-end support wheel  86  are alignment ribs  116 . As may be clearly seen, these alignment ribs  116  are slightly tapered to facilitate easy insertion of the roll-bar-end support wheel  86  into the end of the roll bar  36  during assembly of the apparatus  10  for covering an architectural opening. A smooth barrel  118  is supported at the center of the roll-bar-end support wheel  86  by a plurality of spokes  120 . The left end of the smooth barrel  118  includes an annular bearing surface  122 , which rides in a channel  124  (FIG. 6A) on the inside surface, as depicted, of the skew adjustment plate  66 , adjacent the cylindrical housing  64 . Also visible in FIG. 5A is a complimentary channel  126  and its side walls  128 , which accommodate the elongated channel  110  (FIG. 5B) of the roll bar  36  in the assembled head rail  22 . 
     Referring now to FIGS. 5A and 6A, additional details concerning the skew adjustment plate  66  are provided. The left-hand side of the skew adjustment plate  66 , as depicted, includes the arcuate threaded surface  84  previously described. The cylindrical housing  64  projects from the right side of the skew adjustment plate  66  and is integrally molded in the preferred embodiment with the skew adjustment plate  66 . A bore  132  passes completely through the skew adjustment plate  66  and the center of the cylindrical housing  64 . Referring in particular to FIG. 6A, the right side, as depicted, of the skew adjustment plate  66  includes a substantially annular channel wall  134  defining the substantially annular channel  124 . Two support wheel locks  138  are arranged on the surface of the cylindrical housing  64 . When the roll-bar-end support wheel  86  is slid into position over the cylindrical housing  64  and is fully seated so that the annular bearing surface  122  of the roll-bar-end support wheel  86  is against the skew adjustment plate  66 , the support wheel locks  138 , which are located approximately 180° apart on the surface of the cylindrical housing  64 , snap over the annular ledge  527  visible in FIGS. 5A and 24C to rotatably lock the roll-bar-end support wheel  86  into position. When the roll-bar-end support wheel  86  is thus positioned over the cylindrical housing  64 , the arcuate arm  94  of the down limit stop  88  rides in the substantially annular channel  124  visible in FIG.  6 A. The arcuate arm  94  riding in this channel  124  is also clearly depicted in FIG.  24 A. Locking fingers  140  are molded into the distal end of the cylindrical housing  64  (FIG.  6 A). When the head rail  22  is fully assembled as depicted in FIGS. 3A,  3 B, and  4 , for example, the locking fingers  140  are engaged by the four locking lugs  142  depicted on the left end in FIG.  5 B. 
     Referring now to FIG. 5B, the components of the threaded shaft  72  are described next. In the preferred embodiment, the threads on the threaded shaft are left-handed threads. The left end, as depicted, of the threaded shaft  72  comprises a head  144 . On the interior of the head  144  are the two short interlocking tabs  70 , which engage the interlocking channels  68  on the plunger  46  (see FIG. 5A) after the head rail  22  is assembled. Moving outward radially from the interlocking tabs, an annular abutment surface  146  is next encountered. As may be seen, for example, in FIG. 17, this annular abutment surface rides against the inward side of the spring retention ring  62 . Moving further out radially on the left-hand end, as depicted in FIG. 5B, of the threaded shaft  72 , the four locking lugs  142  are next present. These four locking lugs  142 , which are positioned at substantially 90° intervals around the circumference of the annular abutment surface  146 , engage the locking fingers  140  of the cylindrical housing  64  to facilitate adjustment of the maximum amount of retraction of the covering  12  that is possible. The four locking lugs  142  project leftward, in FIG. 5B, from a finger seat  148 , which is annular in configuration. The reader is referred, for example, to FIG. 19, which shows the locking fingers  140  of the cylindrical housing  64  resting against the finger seat  148  located on the head  144  of the threaded shaft  72  when the head rail  22  is assembled and is not being adjusted. Finally, on the back side, as depicted in FIG. 5B, of the head  144  of the threaded shaft  72  is a stopping ledge  150 . The function of the stopping ledge  150 , which may also be clearly seen in FIGS. 18 and 19, will be described in further detail below. 
     Referring again to FIG. 5B, the next component encountered is the internal, roll-bar-support, wheel  152 . This internal, roll-bar-support wheel  152  may also be seen in at least FIGS. 3A,  3 B,  4 , and  22 . The internal, roll-bar-support wheel  152  includes an internally threaded barrel  154 . This threaded barrel  154  makes it possible to thread the internal, roll-bar-support wheel  152  onto the threaded shaft  72  adjacent the wheel  152  in FIG.  5 B. The threaded barrel  72  is supported by a plurality of barrel support spokes  156  which extend radially between the outer surface of the threaded barrel  154  and the outer ring  157  of the internal, roll-bar-support wheel  152 . The outer ring  157  of this wheel  152  is not completely rounded. In particular, contact ribs  158  are present on the outer surface of the outer ring  157 . When the internal, roll-bar-support wheel  152  is inserted into the roll bar  36 , these contact ribs  158  ride on the inner surface of the roll bar  36  and help ensure that the alignment of the internal, roll-bar-support wheel  152  is correct. Also present on the outer surface of the outer ring  157  is an alignment groove  160 . The alignment groove  160  accommodates the alignment tongue  114  running down the inside of the roll bar  36  parallel to the longitudinal axis of the roll bar  36 . When the internal, roll-bar-support wheel  152  is properly inserted into the interior of the roll bar  36 , the alignment tongue  114  rides in the alignment groove  160 , which helps ensure that the internal, roll-bar-support wheel  152  and the roll bar  36  rotate in unison. The outer ring  157  of the internal, roll-bar-support wheel  152  also includes a complimentary channel  162  and side walls  164 , which accommodate a similar elongated channel  110  and its corresponding channel side walls  165  formed integrally with the roll bar  36 . Thus, when the internal, roll-bar-support wheel  152  is properly inserted into the interior of the roll bar  36 , the alignment tongue  114  is trapped within the alignment groove  160 , and the elongated channel  110  of the roll bar is similarly captured in the complimentary channel  162  in the internal roll-bar-support wheel  152 . Also visible on the internal roll-bar-support wheel  152  depicted in FIG. 5B is an intercepting ledge  166 . If the internal, roll-bar-support wheel  152  is threaded far enough onto the threaded shaft  72 , the intercepting ledge  166  of the roll-bar-support wheel  152  will impact on the stopping ledge  150  of the threaded shaft  72 . This interaction is described further below with reference to FIGS. 18 and 19. 
     Next, depicted in the upper half of FIG.  5 B and in the lower leftmost portion of FIG. 5B are fragmentary portions of the roll bar  36 . The primary features of the roll bar  36 , including the alignment tongue  114  and the elongated channel  110  have been described previously. 
     The remaining components depicted in FIG. 5B (namely the screw  168 , drive member  170 , clutch coil spring  172 , and mounting hub  174 ) cooperate with several components depicted in FIG. 5C to rotatably support the right-hand end, as depicted, of the roll bar  36 . These components include a break away operating cord system  176  substantially identical to that described in co-pending application Ser. No. 09/050,507, filed 30 Mar. 1998, which disclosure is incorporated in the present application as though fully set forth herein. The reader is referred to that prior application for further details concerning the construction and operation of the break away cord mechanism in addition to the disclosure provided in the present application. The drive member  170  (FIG. 5B) includes a generally cylindrical main body  178  having a plurality of generally radial support ribs  180  projecting from an outer surface of the cylindrical main body  178 . One of the support ribs includes an alignment groove  182 , which is similar to the alignment groove  160  previously described in connection with the internal, roll-bar-support wheel  152 . When the drive member  170  is inserted into the right end, as depicted, of the roll bar  36  and is properly aligned, the alignment tongue  114 , which is an integral part of the internal surface of the roll bar  36 , rides in the alignment groove  182 , thereby forcing the drive member  170  and roll bar  36  to rotate in unison. A tapered barrel  184  is suspended by a plurality of barrel support spokes  186  extending between the exterior surface of the tapered barrel  184  and the internal surface of the generally cylindrical main body  178  of the drive member  170 . At the right-hand end, as depicted, of the drive member  170  is a drive wheel  188 . The drive wheel  188  includes alternate radially extending teeth  190 , which define a channel  192  between them. As shown in other figures (e.g., FIG.  8 ), the channel  192  accommodates an operating cord  193 . 
     The tapered barrel  184  suspended in the center of the generally cylindrical main body  178  does not extend the full length of the inside of the generally cylindrical main body  178 . Rather, as is clearly depicted in FIGS. 3A,  3 B, and  4 , for example, the tapered barrel  184  extends only approximately half way through the generally cylindrical main body  178 . Subsequently, the inside of the generally cylindrical main body  178  becomes larger. The diameter of this larger portion of the internal surface of the generally cylindrical main body  178  is designed to accommodate the clutch coil spring  172  depicted in FIG.  5 B. The internal surface of the generally cylindrical main body  178  is merely notched a sufficient amount to accommodate the clutch coil spring  172 . When the clutch coil spring  172  is properly installed, the internal surface of the spring  172  is substantially coplanar with the internal surface of the generally cylindrical main body. 
     A mounting hub  174  is the final component visible in FIG.  5 B. The mounting hub  174  has a central cylindrical axial passage  198  and includes a generally U-shaped longitudinally extending channel  200 . On the right-hand end, as depicted, of the mounting hub  174  is a bearing surface  202 . This bearing surface is substantially annular and rides on the inner ring-like bearing surface  204  (FIG. 5C) located on the inward side of the relatively flat base of the right end cap  26  when the head rail  22  is fully assembled. 
     Even though FIG. 5B shows only one clutch spring  172  in the preferred embodiment there are two clutch springs placed back-to-back in the drive member  170 . 
     Referring now to FIG. 5C, additional components of the right end of the head rail  22  are depicted. First, a releasable mounting plate  206  is shown. This releasable mounting plate  206  includes a generally U-shaped notch  208 . This generally U-shaped notch  208  is defined by side edges  210 ,  210 ′ that extend from the distal end of a pair of clamp arms  212 ,  212 ′ toward a pair of horizontal lips  214 ,  214 ′ and then around an arcuate segment  216  defining an enlarged recess area  218 . This enlarged recess area  218  and the horizontal lips  214 ,  214 ′, conform to the shape molded into the rear side, as depicted, of the mounting hub  196  (see FIG. 6B, which shows the rear side of the mounting hub  174 ). The releasable mounting plate  206  also includes a pair of mounting blocks  220  on the peripheral edges of each clamp arm  212 ,  212 ′. These mounting blocks  220  each define a pulley channel  222  that is substantially U-shaped. A pin hole  224  is located on the legs of the pulley channel and a shaft hole  226  is located in the base of the pulley channel  222 . During assembly, a pulley wheel  228  is mounted in each pulley channel  222  by inserting the shaft  229  of the pulley wheel  228  into the shaft hole  226  of the pulley channel  222 . Then, the operating cord  193  (FIG. 8) is threaded above the pulley wheel  228  between the upper portion of the mounting block  220  and the top of the pulley wheel  228 . Then, the pulley plate  300 , which comprises a pair of mounting pins  302  on its back side  303  and includes a shaft hole on its back side (not depicted) is positioned to rotatably secure the pulley wheel  228  in position in the pulley channel  222 . When the pulley plate  300  is properly positioned over the mounting block  220 , the top side  301  of the pulley plate is substantially coplanar with the top surface  305  of the semi-circular guide plate  304 . 
     The lock plate  306  depicted in FIG. 5C may be used to disable the break-away feature of the operating cord  193 . The lock plate  306  is slid into position after the other components of the break away operating cord system are assembled. When properly positioned, the upstanding legs  308  of the lock plate  306  prevent the two clamp arms  212 ,  212 ′ of the releasable mounting plate  206  from permitting the releasable mounting plate  206  from releasing. Since it may be difficult to remove the lock plate  306  after it has been inserted, the lock plate  306  includes an elongated slot  310 . If the lock plate  306  is difficult to remove, a flat-blade screwdriver may be inserted into the elongated slot  310  to facilitate removal of the lock plate  306 . 
     Various details of the inner surface of the right end cap  26  are visible in FIG.  5 C. Protruding from the relatively flat base  311  of the right end cap  26  is a tapered support shaft  312 . This tapered support shaft  312  supports the mounting hub  174  and the drive member  170  as shown in FIG. 4, for example. Extending substantially parallel to the tapered support shaft is the stop arm  314 . A pair of abutment surfaces  316  are visible on each side of the right end cap  26 . These abutment surfaces  316  are impacted by the abutment surfaces  213  on the clamp arms  212 ,  212 ′, one of which is visible on the releasable mounting plate depicted in FIG.  5 C. Also visible in FIG. 5C is a top wall  318 , which is an integral part of the right end cap  26 . When the head rail  22  is fully assembled, as depicted in FIG. 1, for example, an end portion  400  of the top wall abuts a corresponding surface on the arcuate cover plate  28 . The back side of the arcuate cover plate  28  is supported by the arcuate, plate-like projection  402  depicted in FIG.  5 C. This arcuate, plate-like projection  402  is integrally molded as a part of the right end cap  26  in the preferred embodiment. Finally, a cord guide surface  404  is also depicted in FIG. 5C as being integrally formed on the back side or internal side, as depicted, of the right end cap  26 . 
     When the break away clutch system is completely assembled, it appears as depicted in FIGS. 4,  7 , and  8 , for example. FIG. 7 depicts a cross-sectional view taken along line  7 — 7  of FIG.  4 . Clearly visible in FIG. 7 are the abutment surfaces  213  on each of the clamp arms  212 ,  212 ′ of the releasable mounting plate  206  in proximity to the corresponding abutment surfaces  316  of the right end cap  26 . FIGS. 7 and 8 are included in the present application primarily for context. For additional details and explanation concerning the assembly and operation of the break away clutch mechanism, the reader is referred to co-pending application Ser. No. 09/050,507, which has been incorporated herein by reference. 
     Referring now to FIGS. 9,  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16 , the bottom rail  20  of the present invention is next discussed. The bottom rail  20 , an isometric view of which is clearly shown in FIG. 1, comprises a bottom plate  412 , a compression plate  414 , a pair of end caps  416  and an optional weight  418 . FIG. 9 is a fragmentary cross-sectional view of a portion of the bottom rail  20  taken along line  9 — 9  of FIG.  1 . FIG. 9 depicts the relationship between the left bottom rail end cap  416 , the first and second flexible sheets  14 ,  16 , the compression plate  414 , and the optional weight  418 . As seen in FIGS. 9,  10 , and  11 , the bottom rail end caps  416  (the right end cap is not depicted but is the same as the left end cap) include an upper projection  500  and two lower projections  502  extending from the inside surface  504  of the end caps  416 . The upper projection  500  is shown in phantom in FIG. 9, but additional details concerning the upper projection  500  may be clearly seen in FIGS. 10 and 11. The two lower projections  502  depicted in FIG. 10 extend in the preferred embodiment approximately the same distance from the inside surface  504  of the rail end caps  416  as does the upper projection  500 . These three projections frictionally engage the compression plate  414  and the bottom plate  412  of the bottom rail  20  to removably secure the end caps  416  to the bottom rail  22 . 
     Referring in particular to FIG. 13, the bottom plate  412  is next described. As shown in FIG. 13, the bottom plate has a winged U-shape when viewed in cross-section perpendicular to the longitudinal axis of the bottom rail  20 . Two strips of gripping material  506  extend along the interior surface of the bottom plate  412 . These strips of gripping material  506  are substantially parallel to the longitudinal axis of the assembled bottom rail  20 . When the first and second sheets  14 ,  16  are trapped during bottom sheet assembly (see, for example, FIG.  16 ), the gripping material  506  helps hold the flexible sheet material in position. In the preferred embodiment, the bottom plate  412  itself is made from a plastic material, and the gripping material is a type of gummier, rubber-like material. Extending upward (leftward as depicted in FIG. 13) from the bottom plate  412  and continuing for the entire length of the bottom rail  20  in a longitudinal direction are a pair of inwardly projecting ledges  508 . The ledges  508  project inwardly from a distal end of a vertical wall  509  and are substantially perpendicular to the vertical wall  509 . The vertical walls  509  are attached at one end to the bottom plate  412 . A weight channel  510  is defined by the substantially rectangular pocket created between the undersides of the inwardly projecting ledges  508  and the inside surface of the bottom plate  412 . If the optional weight  418  were used, it is preferably placed in the weight channel  510  as shown in FIG.  15 . The weight  418  may be used to help the covering  12  extend more easily, and the optional weight could also assist in anti-skew adjustment. On the opposite sides of the substantially vertical walls  509 , are two other ledges  516 ,  516 ′ extending toward the longitudinal edges  413  of the bottom plate  412 . Each of these latter two ledges  516 ,  516 ′ also extend for the entire longitudinal length of the bottom plate  412  in the preferred embodiment. Each of these latter ledges  516 ,  516 ′ also interlocks with a corresponding ledge  517 ,  517 ′, respectively, on the compression plate  414  to secure the bottom plate  412  to the compression plate  414 . 
     Referring now to FIG. 12, the compression plate  414  in the preferred embodiment has a substantially arcuate cross-section. A pair of substantially vertical walls  512  extend from the underside of the compression plate  414  and extend for the entire longitudinal length of the compression plate  414  in the preferred embodiment. The distal edges  514  of each of the substantially vertical walls  512  comprises an interlocking ledge  517 ,  517 ′. Each of these interlocking ledges  517 ,  517 ′ corresponds with an interlocking ledge  516 ,  516 ′, respectively, on the bottom plate  412 . In the preferred embodiment, the compression plate  414  is made from aluminum or some similar rigid material, while the bottom plate  412  is made from a flexible plastic material. Thus, when the compression plate  414  is forced toward the bottom plate  412 , the interlocking ledges  516 ,  516 ′ on the flexible bottom plate  412  snap around the interlocking ledges  517 ,  517 ′, respectively, on the substantially rigid compression plate  414 , thereby locking the two components together as shown in FIGS. 14 and 15, for example. 
     Referring now to FIG. 16, the assembly of the bottom plate  412 , compression plate  414 , and the covering  12  is described. As shown in FIG. 16, the first flexible sheet  14  and the second flexible sheet  16  of the covering  12  each has a trailing edge  518  extending below the lowest horizontal vane  18  connecting these two flexible sheets. To attach the bottom rail  20  to the covering  12 , the relatively rigid compression plate  414  is placed between the trailing edges  518  of the first and second flexible sheets  14 ,  16 . Then, the bottom plate  412  is pressed toward the compression plate  414  while ensuring that the trailing edges  518  extending past the compression plate  414  are placed on top of the longitudinally extending strips of gripping material  506  affixed along the longitudinal edges  413  of the bottom plate  412 . With the trailing edges  518  of the two flexible sheets  14 ,  16  positioned as shown in FIG. 16, the bottom plate  412  is pressed toward the compression plate  414  until the first and second interlocking ledge pairs  516 / 517 ,  516 ′/ 517 ′ snap together, as shown in FIG.  15 . When the bottom rail  20  has been properly assembled, the trailing edges  518  of the first and second flexible sheets  14 ,  16  are trapped between the gripping material  506  and the interior surface of the compression plate  414 . 
     Referring now to FIGS. 17,  18 ,  19 ,  20 , and  21 , operation and adjustment of the control system hardware that controls the upper retraction limit is next described. FIG. 17 shows a cross section of the left-hand end of the assembled head rail  22 . As shown in FIG. 17, the plunger  46  is snapped together with the threaded shaft  72 , and the setting retention spring  60  is trapped between the spring retention ring  62  and the underside of the plunger head  48 . Tension within the setting retention spring  60  causes the spring to press against the spring retention ring  62  and the plunger head  48 , thereby biasing the plunger head  48  toward the left, which simultaneously biases the threaded shaft  72  to the left as depicted in FIG.  17 . When the threaded shaft  72  is thus biased to the left, as depicted, this causes the four locking lugs  142  on the head  144  of the threaded shaft  72  (see FIG. 5B) to engage the locking fingers  140  on the distal end of the cylindrical housing  64  of the skew adjustment plate  66  (see FIG. 5A for a clear view of the locking fingers  140 ). When in this configuration, the threaded shaft  72  is kept from rotating by the pressure between the four locking lugs  142  and the locking fingers  140 . Therefore, if the roll bar  36  is rotated in one of the directions indicated by the bent arrows  520 ,  522  at the right side of FIG. 17, this causes the internal roll-bar-support wheel  152  to move left or right, as depicted in FIG. 17, parallel to the axis of rotation  196  of the roll bar  36 . Rotation of the roll bar  36  thus rotates the internal roll-bar-support wheel  152 , which must rotate substantially in unison with the roll bar  36  because of the interaction between the alignment tongue  114  and the alignment groove  160  (visible in FIG. 5B) and interaction between the elongated channel  110  and the complimentary channel  162  (also visible in FIG.  5 B). Since the internal roll-bar-support wheel  152  comprises a threaded barrel  154  that is threaded on the threaded shaft  72 , any rotation of the internal, roll-bar-support wheel  152  results in a proportional longitudinal movement of the internal roll-bar-support wheel  152  as the threaded barrel  154  rotates along the threaded shaft. For example, when the covering  12  is extended (i.e., when the roll bar  36  is rotated in the direction indicated by the arrow  522  in FIG.  17 ), the internal roll-bar-support wheel  152  is driven toward the right as depicted in FIG.  17 . This occurs because in the preferred embodiment, the threaded barrel  154  and the threaded shaft  72  have left-handed threads. Obviously, the length of the threaded shaft  72  is at least partially dependent upon the size of the covering  12  that must be unrolled (i.e., the number of rotations that the internal roll-bar-support wheel  152  will complete during extension of the covering). If the threaded shaft  72  is not sufficiently long, extension of the covering will eventually force the internal roll-bar-support wheel  152  to fall off the right end, as depicted, of the threaded shaft. Of course, one could implant a pin or shaft (not shown) perpendicular to the threaded shaft  72  near its free end in order to prevent the internal roll-bar-support wheel  152  from falling off the right end (as depicted in FIG. 17) of the threaded shaft  72 . Such a pin or shaft that stops the lateral or longitudinal movement of the internal roll-bar-support wheel  152  could act as a backup to the gravity lock disclosed herein and described further below. 
     FIGS. 18 and 19 each shows a fragmentary cross-sectional view along line  18 — 18  of FIG. 17 to demonstrate how the upper stop limit for the covering  12  is set. In FIG. 18, the covering  12  (shown in FIG. 1) is at least partially extended. This is apparent because the intercepting ledge  166  is displaced from the stopping ledge  150  since the internal roll-bar-support wheel  152  is displaced partway down the threaded shaft  72 . As the covering  12  is retracted (i.e., the roll bar  12  is rotated in the direction  520  indicated in FIG.  17 ), the threaded barrel  154  and, thus the internal roll-bar-support wheel  152 , moves to the left in FIGS. 18 and 19 until the intercepting ledge  166  on the edge of the threaded barrel  154  intercepts the stopping ledge  150  on the head  144  of the threaded shaft  72 . When the intercepting ledge  166  intercepts the stopping ledge  150 , no further retraction of the covering  12  may occur. Thus, if the stopping ledge  150  and the intercepting ledge  166  have met, but the covering  12  is not retracted as far as desired, it is necessary to adjust the relative position between the internal roll-bar-support wheel  152  and the threaded shaft  72  to prevent the intercepting ledge  166  from intercepting the stopping ledge  150  until the covering  12  is retracted the desired amount. Adjustment of this relationship between the internal roll-bar-support wheel  152  and the threaded shaft  72  is depicted in FIGS. 20 and 21. 
     FIGS. 20 and 21 show adjustment of the relative position of the internal roll-bar-support wheel  152  relative to the threaded shaft  72 . Referring first to FIG. 20, a screwdriver  524  is shown inserted in the screwdriver slot  58  (FIG. 5A) in the plunger head  48 . In order to gain access to the screwdriver slot, the access door  32  (visible in FIGS. 1 and 5A) has been removed, and the screwdriver  524  has been inserted through the access port  42  in the left end cap  24 . When the screwdriver  524  is forced with sufficient pressure into the screwdriver slot  58  in the plunger head  48 , this action compresses the setting retention spring  60  as the plunger  46  travels rightward as depicted in FIG.  20 . The plunger  46  and the threaded shaft  72  move in unison because of the interaction among several components, including the intermediate cylindrical portion  52  of the plunger, the interlocking channels  68  on the intermediate cylindrical portion  52 , the locking tabs  74  on the flexible arms  56 , the interlocking tabs  70  on the interior of the head  144  of the threaded shaft  72 , and the annular abutment surface  146  on the left end (as depicted in FIG. 5B) of the threaded shaft  72 . Thus, when the plunger  46  is driven rightward in FIG. 20, this simultaneously disengages the locking lugs  142  of the threaded shaft  72  from the interlocking fingers  140  of the cylindrical housing  64  of the skew adjustment plate  66  after the setting retention spring  60  has been compressed a sufficient amount. Once the interlocking lugs  142  are thus disengaged from the locking fingers  140 , rotation of the screwdriver  524  directly rotates the threaded shaft  72 . Thus, if the roll bar  36  remains motionless, this rotation of the threaded shaft  72  will force the internal roll-bar-support wheel  152  to move left or right, depending upon the direction of rotation of the screwdriver  524 . For example, if the screwdriver  524  is rotated in a first direction  523  while the roll bar  36  is kept from moving, the internal roll-bar-support wheel  152  will be pulled to the left in FIG. 20 by the interaction between the threads of the threaded barrel  154  and the threads on the threaded shaft  72 . Similarly, if the screwdriver  524  is turned in the second direction  525  while the roll bar  36  is prevented from rotating, the internal roll-bar-support wheel  152  will be pushed to the right in FIG. 20 by the interaction between the threaded barrel  154  and the threaded shaft  72 . By making these adjustments, which increase or decrease the number of threads between the left edge of the internal roll-bar-support wheel  152  and the head  144  of the threaded shaft  72 , it is possible to adjust the number of rotations that the roll bar  36  is permitted to go through before the intercepting ledge  166  on the internal roll-bar-support wheel  152  intercepts the stopping ledge  150  on the back side of the finger abutment ring  149  on the head  144  of the threaded shaft  72 . When the pressure driving the screwdriver  524  rightward in FIG. 20 is released, the setting retention spring  60  drives the plunger  46  and threaded shaft  72  to the left in FIG. 20 until the four locking lugs  142  engage locking fingers  140  on the cylindrical housing  64 , and the tips of the locking fingers  140  rest against the finger seat  148  (FIG. 5B) of the finger abutment ring  149 . Once the interlocking lugs  142  are locked into the locking fingers  140 , the threaded shaft  72  again becomes effectively fixed to the left end cap  24  and, thus, remains stable during rotation of the roll bar  36 . FIG. 21 is a fragmentary view taken along line  21 — 21  of FIG.  20  and depicts disengagement of the locking lugs  142  (two of which are depicted) from the locking fingers  140 . 
     FIG. 22 is a partial cross-sectional view taken along line  22 — 22  of FIG.  20  through the center of the internal roll-bar-support wheel  152 . The threaded barrel  154  of the internal roll-bar-support wheel  152  is shown as threaded onto the threaded shaft  72 , the edge of the threads shown in phantom as a ring around the threaded shaft  72 . Placement of the internal roll-bar-support wheel  152  within the roll bar  36  is also clearly visible in FIG.  22 . The alignment tongue  114  is shown as riding in the alignment groove  160 , and the complimentary channel  162  of the internal roll-bar-support wheel  152  is shown accommodating the elongated channel  110  built in to the roll bar  36 . The wedge  92  of the down limit stop  88  is also visible riding on the outside of the roll bar  36  in the elongated channel  110 . The threaded barrel  154  is supported by a plurality of barrel support spokes  156 . Although spokes  156  are used in the preferred embodiment, clearly the spokes  156  could be replaced by solid material or the number of barrel support spokes  156  could be increased or decreased at the whim of the designer. Several layers of the covering  12  are shown as still being wound around the roll bar  36  in FIG. 22, and a portion of the covering  12  has been unwound and is hanging down from the right-hand side, as depicted, in FIG.  22 . 
     Referring now to FIGS. 23A,  23 B,  24 A and  24 B, operation of the extension limit (gravity lock) in the present invention is described next. FIG. 23A is a fragmentary cross-sectional view taken about line  23 A— 23 A in FIG.  4 . Clearly visible in FIG. 23A is the left end cap  24 , the arcuate cover plate  28 , a portion of the roll bar  36 , the roll-bar-end support wheel  86  with the down limit stop  88  (FIG. 5A) mounted thereon, and a portion of the covering  12 . As shown by the direction arrow  91  in FIG. 23A, the roll bar  36  is rotating clockwise and extending the covering  12  comprising the first flexible  14 , the second flexible sheet  16 , and the horizontal vanes  18 . As depicted in FIG. 23A, the covering  12  is nearing complete extension. The interior side of the first flexible sheet  14  is pressing against the outer surface  102  of the wedge  92  on the down limit stop  88 , thereby keeping the wedge  92  from rotating about its mounting tang  90 . FIG. 24A shows the covering and roll bar  36  in approximately the same position from the opposite direction since FIG. 24A is a partial cross-sectional view taken about line  24 A— 24 A in FIG.  4 . In FIG. 24A it is clearly visible that the flexible sheet  14  pressing against the outer surface  102  of the wedge  92  is keeping the arcuate arm  94  within the semi-annular channel  124  (see also FIG. 6A) defined between the semi-annular channel wall  134  and the annular bearing surface  122  (FIG. 5A) on the roll-bar-end support wheel  86 . FIG. 23B is similar to FIG. 23A; however, rotation of the roll bar  36  has been stopped by the down limit stop  88  and the covering  12  is in its fully extended configuration. When the roll bar  36  rotates from the position shown in FIG. 23A to that shown in FIG. 23B, no covering material remains on the roll bar  36  to press against the outer surface  102  of the wedge  92  and keep the down limit stop  88  from rotating about the mounting tang  90 . Therefore, shortly after being in the position shown in FIG.  23 A and shortly before reaching the position shown in FIG. 23B, gravity causes the down limit stop  88  to rotate about its mounting tang  90  to the position shown in FIG.  23 B and in FIG. 24B, which shows the same position from the opposite side. With the down limit stop  88  thus rotated, the leading edge  100  of the arcuate arm  94  impacts the edge of the semi-annular channel wall  134  since the arcuate arm  94  of the down limit stop  88  is no longer forced to remain within the semi-annular channel  124  by the pressing of the covering material on the outer surface  102  of the wedge  92 . When the leading edge  100  of the arcuate arm  94  impacts the semi-annular channel wall  134 , as depicted most clearly in FIG. 23B, the trailing edge  106  of the wedge  92  is simultaneously driven into a side wall  165  of the elongated channel  110  in the roll bar  36 . Thereby, any further downward motion of the covering  12  toward the extended position is prevented. When the roll bar  36  is rotated in the opposite direction to that depicted by the direction arrow  91  in FIG. 23A in order to retract the covering  12  by winding it back on to the roll bar  36 , the opposite edge  135  (FIG. 24B) of the semi-annular channel wall  134  impacts the outer edge  98  of the arcuate arm  94 , thereby rotating the down limit stop  88  counterclockwise as depicted in FIG. 24B about the mounting tang  90  and pushing the arcuate arm  94  back into the semi-annular channel  124  defined between the semi-annular channel wall  134  and the annular bearing surface  122  of the roll-bar-end support wheel  86 . Then, as the roll bar  36  continues to retract the covering  12  and completes its first full rotation, the down limit stop  88  is prevented from rotating about its mounting tang  90  since a layer of the covering  12  will then be present to press against the outer surface  102  of the wedge  92  during further retraction of the covering  12 . 
     FIG. 24C is a fragmentary cross-sectional view taken about line  24 C— 24 C of FIG.  24 B. This figure clearly shows how the support wheel locks  138 , which in the preferred embodiment is an integral part of the cylindrical housing  64  on the skew adjustment plate  66  (see, e.g., FIG.  6 A), snap behind the annular ledge  527  on the inside of the otherwise smooth barrel  118  suspended in the center of the roll-bar-end support wheel  86  by a plurality of spokes  120 . When the roll-bar-end support wheel  86  is slid onto the cylindrical housing  64  of the skew adjustment plate  66 , the support wheel locks  138  are flexed toward the axis of rotation  196  of the roll-bar-end support wheel  86  until the roll-bar-end support wheel  86  is slid sufficiently far onto the cylindrical housing  64  that the support wheel locks  138  can trap the support wheel  86  onto the cylindrical housing  64  by springing out behind the ledge  527 . Also clearly visible in FIG. 24C is the method of attaching the down limit stop  88  to the roll-bar-end support wheel  86 . When the mounting tang  90  is pushed sufficiently into the mounting hole  108  on the support wheel  86 , the locking tabs  96  on the distal end of the mounting tang  90  snap past a ridge  526  on the inside of the mounting hole  108  where the mounting hole diameter increases slightly. 
     Referring next to FIGS. 3B,  5 A,  6 A, and  25 , the control system components that permit one type of skew adjustment available with the present invention are described next. As shown in FIG. 3B, if the left end cap  24  is incorrectly mounted higher than the right end cap  26 , for example, a skew angle  528  will be present between an imaginary horizontal line  530  and a second imaginary line  532  extending between the top of the right end cap  26  and the top of the left end cap  24 . This skew angle  528  can be compensated for or corrected by turning the threaded skew adjustment plug  76  in the plug bed  78  (FIG. 6A) by inserting a screwdriver  524  (FIG. 3B) through the access hole  80  (most clearly visible in FIG.  6 A). When the skew adjustment plug  76  is rotated, the threads on the skew adjustment plug  76 , which engage the arcuate threaded surface  84  (FIGS.  5 A and  3 B), molded into the skew adjustment plate  66 , drive the skew adjustment plate  66  upward or downward, depending on the direction of rotation of the skew adjustment plug  76 . The skew adjustment plate  66  is capable of moving up and down relative to the left end cap  24  since the front vertical edge  534  and the rear vertical edge  536  (see FIG. 6A) of the skew adjustment plate  66  ride in complimentary channels  82  molded onto the interior surface of the left end cap  24  (FIG.  6 ). Since the cylindrical housing  64  of the skew adjustment plate  66  moves the axis of rotation of the roll bar  36  via the interaction between the cylindrical housing  64 , the roll-bar-end support wheel  86 , and the roll bar  36 , as the skew adjustment plate  66  is driven upward or downward by rotation of the skew adjustment plug  76 , the entire left end (as depicted in FIG. 3B) of the roll bar  36  moves upward or downward. It is thereby possible to position one end of the roll bar  36  relative to the other end of the roll bar  36  without having to move the end caps  24 ,  26 , which may be fixed relative to a mounting surface by mounting brackets  30  (see FIG.  1 ). FIG. 25 provides a view of the skew adjustment plate  66  in position in the channels molded on the inward surface of the left end cap  24 . The skew adjustment plug  76  is pinched between the arcuate threaded surface  84  of the skew adjustment plate  66  and the plug bed  78  (FIG. 6A) of the left end cap  24 . The skew adjustment plug  76  is pinched with sufficient pressure that the skew adjustment plate  66  will not move due merely to the weight of the roll bar  36  and covering  12 , but the skew adjustment plug  76  is not pinched so hard that desired skew adjustment is difficult to achieve. 
     Although preferred embodiments of this invention have been described above, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. For example, each of the support wheels  86 ,  152  could be made with more or fewer spokes or they could be made with no spokes to support the central barrels, whether threaded or unthreaded. Also, in the preferred embodiment, the threaded shaft  72  and the threaded barrel  154  in the internal-roll-bar support wheel  152  are left-hand threaded. If desired, a right-hand thread could be used, but the covering  12  may be required to roll on the roll bar  36  from the opposite side from that depicted in the enclosed drawings, or the control system components that make it possible to control the maximum retraction and maximum extension of the covering could be incorporated into the right-hand end of the head rail  22 . In the break away operating cord system depicted in the present application, a single clutch coil spring  172  is shown in FIG. 5B, but more than one clutch coil spring could be incorporated into this portion of the control system without deviating from the scope of the present invention. The applicant has obtained favorable results from using two clutch coil springs. Also, as depicted in the drawings and discussed above, the covering  12  comprises two flexible sheets  14 ,  16  with a plurality of horizontal vanes  18  extending between them. Any type of roll up covering, however, could be used in conjunction with the control system components of the present invention. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting.