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TECHNICAL FIELD 
   This invention relates generally to fenestration and more particularly to compound windows and doors formed from two or more individual window or door units joined together or mulled to create a larger multi-unit fenestration assembly. 
   BACKGROUND 
   Compound fenestration units, commonly referred to as mulled fenestration units, are formed by joining two or more individual window or door units, which will hereinafter be referred to as component units, so as to form a combination of windows, or windows and doors, that can be handled and installed as a single unit, and which give the appearance of being a single unit. A simple system for joining the component units involves the placing of spacer boards between the units to be joined and installing screws or other fasteners through the frames of the component units, into the spacer boards, to join the units. Other systems for joining the units involve the use of interlocking brackets or other like devices that can be separately installed on the facing surfaces of the frames to be joined and then coupled together to form the compound unit. 
   An important aspect of compound fenestration units is that a great variety of different compound fenestration units can be formed from a relatively limited set of component units. Assembly of component window or door units into compound fenestration units involves not only mechanical coupling of the component window units, but also sealing of the joints between the component units against rain, wind, and other intrusions. Additionally, it is preferred that any sealing system accommodate a variety of gap arrangements and provide a suitable appearance to the compound unit. Silicone RTV, for example, can provide effective sealing for virtually any gap arrangement, either by itself or in combination with weather stripping or other covering or trim pieces, but the appearance of the sealed unit may be less than desirable, and may not provide the desired appearance of a single integrated unit. Additionally, the skill and equipment needed for the proper application of silicone or other like sealants may not always be readily available in all manufacturing settings. More visually pleasing sealing methods, such as preformed gaskets or trim materials can suffer, on the other hand, from a lack of adaptability to different combinations of component window units. There thus is a continuing need for a method and apparatus for joining together individual window units or door units to form multi-unit fenestration assemblies that addresses the problems and shortcomings of the prior art. It is to the provision of such that the present invention is primarily directed. 
   SUMMARY OF THE INVENTION 
   A system for creating compound fenestration units having sealed interfaces between the component units is disclosed. Briefly described, the system includes coupling structures for quickly and conveniently connecting component units to form robust compound units, as well as a sealing system for sealing the interfaces between the component units. 
   The coupling structures provide coupling members that are attached to component units and then coupled to one another by interlocking channels and tabs. In one embodiment, the coupling members extend along the edge of the component units to be joined, and may extend beyond the edges, from one component unit to another, so as to reinforce the compound unit. In another embodiment, the coupling members are relatively discrete components, several of which are attached at various points along the edges of the various component units. The coupling structures also control the spacings between the component units so as to cooperate with a system of sealing components provided for sealing the gaps between the component units. 
   The sealing system is of a dual seal type, with exterior, or shielding seals, and interior, or pressure seals, wherein the interseal cavities between the shielding seals and the pressure seals are provided with drain passages to convey water to a harmless location, such as the exterior of the structure in which the unit is installed. The seals are supported by a low shrink, dimensionally stable material, such as aluminum, so as to form a lineal sealing stock that is compressible in a transverse direction to allow insertion into gaps between component units, yet sufficiently rigid to urge the seals into sealing contact with the surfaces against which they are to seal. As used herein, the term lineal will refer to an elongated structure having a constant cross section over its length. Examples of lineals include stock materials of indefinite length, and components of a specific length that may, in addition, have specific end configurations to enable them to fit with other surfaces. The system of the present invention includes lineal sealing stock material for vertical gaps between component units and lineal sealing stock having an additional drip edge for sealing horizontal gaps between component units. The system further comprises end sealing components that cooperate with the pressure seals as well as with the shielding seals to provide pressure sealing where needed and ventilation and drainage where needed. The invention will be better appreciated upon review of the detailed description set forth below in conjunction with the accompanying drawing figures, which are briefly described as follows. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is an elevation view of a compound fenestration unit. 
       FIG. 2  is an embodiment of a system for joining component units to form a compound fenestration unit. 
       FIG. 3  is a compound fenestration unit joined in the manner portrayed in  FIG. 2 . 
       FIG. 4  is a cross sectional view of the joint connecting the component units portrayed in  FIGS. 1-3 . 
       FIG. 5  is a cross sectional view of an embodiment of a channel and tab joining structure, prior to joining. 
       FIG. 6  is the channel and tab joining structure portrayed in  FIG. 5  in an intermediate position in preparation for joining. 
       FIG. 7  is the channel and tab joining structure portrayed in  FIGS. 5-6  after joining but prior to installation of wedging screws. 
       FIG. 8  is the channel and tab joining structure portrayed in  FIGS. 5-7  after installation of wedging screws. 
       FIG. 9  is a cross sectional view of a first coupling member for an alternative embodiment of a coupling system for connecting component units. 
       FIG. 10  is an elevation view of the first coupling member portrayed in  FIG. 9 . 
       FIG. 11  is a cross sectional view of a second coupling member for an alternative embodiment of a coupling system for connecting component units. 
       FIG. 12  is an elevation view of the second coupling member portrayed in  FIG. 11 . 
       FIG. 13  shows the first and second coupling members portrayed in  FIGS. 9-12  positioned for sliding into the coupling position. 
       FIG. 14  is an elevation view of the assembled joining system portrayed in  FIGS. 9-13 . 
       FIG. 15   a  is a cross sectional view of the joint formed by the coupling system portrayed in  FIGS. 9-14 . 
       FIG. 15   b  is an elevation view of a compound fenestration unit joined by the joining system portrayed in  FIGS. 9-15   a.    
       FIG. 16  is a cross sectional view of a backbone portion of an embodiment of a vertical sealing strip according to the present invention. 
       FIG. 17  is a cross sectional view of an embodiment of a vertical sealing strip. 
       FIG. 18  is a cross sectional view of a joint in a compound fenestration unit sealed by the sealing strip portrayed in  FIG. 17 . 
       FIG. 19  is a cross sectional view of a backbone portion of an embodiment of a horizontal sealing strip. 
       FIG. 20  is cross sectional view of an embodiment of a horizontal sealing strip. 
       FIG. 21  is a cross sectional view of a horizontal joint sealed with the sealing strip portrayed in  FIG. 20 . 
       FIG. 22  is an embodiment of a sealing component for sealing ends of gaps between component units, and for sealing gaps between nailing flanges in compound fenestration units. 
       FIG. 23  is a compound fenestration unit utilizing the sealing component portrayed in  FIG. 22 . 
       FIG. 24  is an end cover for a vertical sealing strip. 
       FIG. 25   a  is a compound fenestration unit utilizing the end cover portrayed in  FIG. 24  to seal the top end of a vertical sealing strip. 
       FIG. 25   b  is a compound fenestration unit utilizing the end cover portrayed in  FIG. 24  for sealing the bottom end of a vertical sealing strip. 
       FIG. 26  is a view of a portion of a compound fenestration unit comprising a gusset plate. 
       FIG. 27   a  is an embodiment of a junction seal for sealing junctions in gaps in a compound fenestration unit. 
       FIG. 27   b  is a cross sectional view of the junction seal portrayed in  FIG. 27   a.    
       FIG. 28  is a cutaway view of the junction seal portrayed in  FIGS. 27   a - 27   b , installed in a compound fenestration unit. 
       FIG. 29  is a partial view of a compound fenestration unit including junction seals and a horizontal sealing strip. 
       FIG. 30  is an exploded view of an embodiment of an end sealing system for a horizontal sealing strip. 
       FIG. 31  is a partial front view of the end sealing system portrayed in  FIG. 30 , after installation of the seals. 
       FIG. 32  is a cross sectional view of the end sealing system portrayed in  FIG. 31 . 
       FIG. 33  is a cross sectional view of the top portion of a fenestration unit, including an embodiment of a drip edge. 
       FIG. 34  is a cross sectional view of the drip edge portrayed in  FIG. 33  prior to installation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  portrays a compound fenestration unit  10  made up of component units  2 ,  4 ,  6 , and  8 , joined at their edges in a way that provides a single integrated unit. As used herein, the edge of a fenestration unit will refer to the surfaces that face one another when component units are joined into compound units. The plane of a fenestration unit will refer to the plane of the pane or other glazing unit. The interfaces between the units include horizontal gap  5  and vertical gap  7 , which cross at gap junction  9 , wherein each of gaps  5  and  7  have a predetermined width. Each component unit is provided with nailing flanges such as  13 ,  14 ,  16 , and  18 . Nailing flanges on the component units may be integral with each component unit, so as to completely surround the unit, in which case the portions of the nailing flanges on mating sides of the units to be joined are removed prior to assembly of the compound unit, leaving the peripheral portions of the nailing flanges for the compound unit. Alternatively, nailing flanges may be provided as separate parts, in which case they may be cut to length from stock material and installed on the outer periphery of the compound unit after assembly of the unit. 
   Joining of the component units can be accomplished in a variety of ways. In the example shown in  FIG. 2 , sashes, jamb liners, and other window component unit parts have been removed, to allow access to frames  26  and  28 , so that they can be attached to spacer board  24  by screws  23 . As shown in  FIG. 3 , the thickness of board  24  determines the spacing between the units, in particular the spacing between sealing faces  27  and  29 , so as to define gap  32 . Referring to  FIG. 4 , sealing faces  27  and  29  are typically formed by exterior trim cladding layers  43  and  45 , which can be made from, for example, polymeric materials such as PVC, or from aluminum. 
   The steps of removing sashes and other parts from component window units prior to assembly into compound units, and then later replacing them, can be inconvenient and time-consuming. This step can be eliminated by the use of coupling systems that comprise a first coupling member that attaches, by external attachment means, to a first component unit, and a second coupling member that attaches, by external attachment means, to a second component unit, without the need to remove internal parts of the component units. The first and second coupling members are then interengaged with one another, thereby coupling the two component units together. The interengagement can utilize, for example, channels and tabs, wherein the tabs of one coupling member are received by the channels of the other coupling member and are locked in place by a clamping or wedging means. More particularly, a first coupling member may comprise a channel opening in a direction perpendicular to the plane of the unit, toward, for example, the exterior side of the unit, and the second coupling member may comprise a tab located in such a way as to be received by the channel in the first coupling member. It is useful for the coupling structures carrying the channels and tabs to be continuous lineal members that extend the full length of the mull. In some cases, this will mean that the coupling members will extend beyond a first unit to a second unit, in which case the coupling member will act as a reinforcement for the overall stiffness of the compound unit. The connection between the two coupling members can be made more rigid by adding an additional channel and tab coupling combination in a location at a suitable distance from the first channel and tab combination. The channel and tab couplings can be locked in place by addition of a wedging device to urge the tab against one wall of the channel. In one embodiment, a wedging screw has been found to be a useful device for locking the coupling members to one another. The wedging screw can be inserted through a hole in the bottom of the channel, parallel to the tab, to wedge itself between the tab and the wall of the channel so as to urge the tab against the wall. The screw can be a thread forming screw to enable it to secure itself in place by partially threading the channel wall or the side of the tab, or both. An embodiment of tab and channel couplings is shown in  FIG. 5 . 
   Referring to  FIG. 5 , an embodiment of a channel and tab joining structure with a wedging screw is portrayed. The joining structure is made up of first mull coupling member  52 , attached to first component unit  501 , and second, cooperating, mull coupling member  56 , attached to a second component unit  502 . First coupling member  52  comprises tab  53  and channel  54 , joined by base plate  55 . Second mull coupling member  56  comprises channel  58  and tab  57 , joined by base plate  59 . Positioning of coupling member  52  relative to component unit  501  is determined by alignment channel  520  in unit  501 , which receives alignment and load transfer tabs  521  and  522  of first mull coupling  52 . In like manner, alignment channel  560  of unit  502  receives alignment and load transfer tabs  561  and  562  of second mull coupling member  56 . Tabs  521 ,  522 ,  561 , and  562  serve not only as locators, but also serve to transfer mechanical loading from coupling members  52  and  56  to component units  501  and  502 , respectively, thereby reducing the dependence on screws  523  for coupling of the component units. It will be appreciated that although alignment channels  520  and  560  provide the tab receiving features for the present embodiment, other tab receiving features, such as narrow kerfs, could also be used. Coupling members  52  and  56  can be produced by stamping and bending or roll forming of sheet metal stock, as would be apparent to one skilled in the art. Coupling members  52  and  56  are attached to component units  501  and  502  respectively by screws  523 , or by other suitable fasteners, as would be apparent to one skilled in the art. 
   Referring again to  FIG. 5 , the component units can be conveniently joined by first placing them on flat surface  50  and lifting unit  502  a distance d. The units are then brought together so that tab  53  of first mull coupling member  52  approaches base plate  59  of second mull coupling member  56  and tab  57  of second mull coupling member  56  approaches base plate  55  of first mull coupling member  52 , as shown in  FIG. 6 . Referring to  FIG. 7 , component unit  502  is then lowered, engaging tab  53  with channel  58  and simultaneously engaging tab  57  with channel  54 . The coupling formed by the combination of coupling members  52  and  56  is then locked by a series of wedging screws  83 , shown in  FIG. 8 . Holes for receiving screws  83  can be predrilled or prepunched prior to assembly. Fixturing may be useful during the installation of screws  83  to prevent movement of coupling member  56  relative to coupling  52  during installation of screws  83 . It may also be useful to attach gusset plates or other reinforcing members to hold the components in more firmly fixed positions relative to one another both during and after assembly. After the component units have been joined, a gap  86  is defined by first sealing face  82  and second sealing face  84 . Additionally, gap  86  may contain first anchoring kerf  503  and second anchoring kerf  504  for receiving anchoring portions of a mull sealing member. 
   Mull coupling members  52  and  56  may be provided as lineal members that extend along the full edges of the component units, and may also extend beyond a single component unit to adjacent component units. They may extend the full height or width of the compound unit, so as to act as a reinforcing structure for the compound unit. More particularly, in the compound unit portrayed in  FIG. 1 , coupling members can extend from the bottom of bottom units  6  and  8  to the top of top units  2  and  4 , thereby providing additional reinforcement to the compound unit. Alternatively, a horizontal coupling member could extend the full length of horizontal gap  5 , from the left sides of units  2  and  6  to the right sides of units  4  and  8 , so as to bridge the component units in the horizontal direction. 
   Referring to  FIGS. 9-15 , an alternative embodiment of a coupling system for connecting component units is portrayed. In this embodiment, the coupling members are relatively short discrete components placed at suitable intervals along the edges of component window units to be joined, rather than being continuous coupling members, as disclosed in the previous embodiment. In this embodiment, first coupling member  900  comprises a base plate  902 , as portrayed in  FIG. 9 , from which protrude alignment and load transfer tabs  903  and  905  in a first direction, and from which further protrude channel base portions  906  and  908 , at edges  915  and  917 , respectively, in a second direction. Lip portions  907  and  909  are attached to channel base portions  906  and  908  to form first channel  912  and second channel  914  respectively. While the various parts of side mull coupling member  900  are described as separate entities, it will be apparent to one skilled in the art that coupling member  900  can be produced as a single part, by, for example, stamping and bending of sheet metal. The formation of alignment and load transfer tabs  903  and  905  can be aided by first forming aperture  923 , shown in  FIG. 10 , and then bending suitably punched tabs to form alignment and load transfer tabs  903  and  905 . Referring again to  FIG. 10 , a side elevational view of first mull coupling member  900  shows a typical length to height aspect ratio of first mull coupling member  900 , as well as screw holes  1023  for attachment to component unit frames. 
   Second side mull coupling member  1100 , portrayed in  FIGS. 11-12 , comprises base plate portion  1102 , from which protrude alignment and load transfer tabs  1103  and  1105  in a first direction, and from which protrude spacer portions  1107  and  1109  in a second direction. Referring to  FIG. 12 , guide tabs  1110 ,  1112 ,  1114 , and  1116  are attached to spacer portions  1107  and  1109 , to act as insertion guides during assembly of compound units. Referring again to  FIG. 12 , a side elevational view of second mull coupling member  1100  shows a typical length to height aspect ratio of second mull coupling member  1100 , as well as screw holes  1223  for attachment to component unit frames. 
   Referring to  FIGS. 13 and 14 , first coupling tab  1107  and second coupling tab  1109  of second mull coupling member  1100  slide into first channel  912  and second channel  914 , respectively, of first mull coupling member  900 , to form complete coupling unit  1400 , as shown in  FIG. 14 .  FIG. 15   a  shows a cross sectional view of a completed coupling of two component units, wherein first mull coupling member  900  is attached to a first window frame portion  1502 , and second mull coupling member  1100  is attached to a second frame portion  1504 , with each coupling being located relative to its respective component unit by alignment and load transfer tabs  903  and  905  of first coupling member  900  that fit into channel  1503  of first frame portion  1502  and alignment and load transfer tabs  1103  and  1105  that fit into channel  1505  of second frame portion  1504 . 
   Referring to  FIG. 15   b , gap width x can be controlled more precisely if spacer shims  1541  and  1542  are placed between coupling unit  1400  along gap  1507  between frame portions  1502  and  1504 . It is preferred that the thickness of the shims allow a snug to slightly compressed fit between frame portions  1502  and  1504 . It will also be apparent that the width of the shims should be chosen so as not to interfere with other components of the compound unit, such as mull sealing strips. Since the spacer shims are only used to maintain spacing x by supporting a relatively small compressive load, and do not serve a coupling function, the choice of suitable materials is relatively wide. Particularly useful materials for the spacer shims are rigid polymeric foams, such as polystyrene or polyurethane foam. Polymeric foams have the additional advantage of being good heat insulators. 
   While the coupling systems disclosed hereinabove enable component units to be mechanically joined into compound glazing units, there is also a need to provide sealing of the joints between the component units against wind, rain, and other intrusions. For this purpose, a system of sealing strips and end seals is provided. In the embodiment shown in  FIGS. 16-18 , a sealing strip particularly useful for sealing vertical gaps comprises a lineal backbone  1600 , having the cross section shown in  FIG. 16 . Support  1600  comprises base portion  1601  having longitudinal edges  1610  and  1620 , to which are attached first leg portion  1602  and second leg portion  1604 . Hook portions  1605  and  1606  may further be attached to distal edges  1608  and  1609  of leg portions  1602  and  1604 , respectively. Support  1600  is compressible in transverse direction  1621 , so that legs  1602  and  1604  can be readily moved toward one another during, for example, installation of the sealing strip. While portions  1601 ,  1602 ,  1604 ,  1605 , and  1606  have been described as separate entities, in practice they will typically be made as a single part, by, for example, forming from a sheet metal strip. Sealing at the top and bottom of a mull strip typically depends on the ends of the strip fitting snugly against end sealing components. For this reason, any significant shrinkage in the sealing strip adds to the risk that an end seal may fail, and leakage may occur. It is therefore preferred that support  1600  be made of a low shrink material, such as aluminum, in particular an aluminum alloy such as  5052  alloy, which is formulated for reduced corrosion. A convenient method of making the support portion is to form a strip of aluminum sheet by bending or roll forming. 
   It is also useful for support member  1600  to be precoated or primed with an adhesion promoting, anti-corrosive, material, such as a chromate pigment in a polymeric binder. Such coatings are commercially available, and their selection and use would be apparent to one of ordinary skill in the art. Polyurethanes are particularly useful as binders for the coating. 
   It will also be recognized that other backbone materials may provide sufficiently low shrink. For example, pultruded or otherwise reinforced polymeric materials may be suitable in some applications. Also, thermosetting polymeric materials may provide useful reductions in shrink, compared to thermoplastic materials. As will be recognized by one of ordinary skill in the art, the allowable shrink will depend on the ability of the end seals to accommodate dimensional changes without allowing leakage. Therefore, suitable shrinkage levels are those that are sufficiently low to be effective in maintaining the seals at the ends of the sealing strip, for the type of end seals being used, under conditions normally encountered by fenestration units. 
   The present invention utilizes a dual sealing system, comprising exterior, or shielding seals, and interior, or pressure seals. The spaces between the exterior and interior seals will be referred to as interseal cavities. The interseal cavities have openings that allow drainage and ventilation, but which are shielded from direct wind. The interseal cavities serve as quiescent dry zones where, under conditions of wind and rain, only a limited amount of rain water enters, due to the shielding effects of the shielding seals and other shielding devices covering the openings. The pressure seals, on the other hand, are complete seals that seal the interseal cavities from the interior of the building. Since the interseal cavities contain little or no water, any leakage of the pressure seals is likely be leakage of air only, which would be unlikely to harm the interior of the building. Moreover, since the pressure seals are protected from weathering and mechanical damage by the shielding seals, the effectiveness of the pressure seals is likely to remain high over an extended period of time. 
   Referring to  FIG. 17 , a dual sealing system comprising several conformable seals attached to support member  1600  is portrayed. Shielding fins  1712  and  1722  are attached to support  1600  at longitudinal edges  1610  and  1620 . Pressure seals  1732  and  1734  are attached to legs  1602  and  1604 , respectively. It has been found useful to make seals  1732  and  1734  tubular in cross section and somewhat inclined toward the exterior side, for ease of installation of sealing strip  1700 , combined with effective sealing. Sealing strip  1700  further comprises cross member  1703 , which divides it into an exterior portion and an interior portion. The exterior portion of sealing strip  1700 , that is to say the portion facing the exterior of the structure in which the compound unit is installed, is the portion of the sealing strip between base portion  1601  and cross member  1703 , while the interior portion is the portion facing the interior of the structure, that is to say the portion between cross member  1703  up to and including hook portions  1605  and  1606 . Cross member  1703 , base portion  1601 , and the portions of legs  1602  and  1604  between cross member  1703  and base portion  1601  collectively define cavity  1707 , which is open at the bottom end to allow any water that may be present to be conveyed to a harmless exterior location. Centering and consistent compression of sealing strip  1700  in the gap to be sealed is aided by ribs  1742  and  1744 , as well as by ribs  1746  and  1748 . 
   Referring to  FIG. 18 , sealing strip  1700  is installed in gap  86 , with hooks  1605  and  1606  engaging kerfs  503  and  504 . Kerfs  503  and  504  provide stops for strip  1700 , and help to orient it relative to gap  86 . Kerfs  503  and  504  also provide additional assurance that strip  1700  will not be unintentionally removed from gap  86 . Shielding fins  1712  and  1722  fit against sealing faces  82  and  84 , respectively, to form a shielding seal. Inner seals  1732  and  1734  also fit against sealing faces  82  and  84 , respectively, to form a pressure seal, thereby forming interseal cavities  182  and  184 . Interseal cavities  182  and  184 , as well as cavity  1707 , are able to drain any water that may be present to a harmless location. They may also be ventilated at the top by shielded ventilated apertures. The seals formed by fins  1712  and  1722 , along with the various shielding components at the top and bottom ends, are often sufficient to prevent leakage. However, under some conditions, such as severe cases of wind and rain, some water may enter interseal cavities  182  and  184 . Since the air in cavities  182  and  184  can be expected to be relatively quiescent, however, any leaked water is likely to drop to the bottom of these cavities, where it can drain out through openings  2510  and  2520 , as shown in  FIG. 25   b . Pressure seals  1732  and  1734  perform the primary sealing function, since the primary part of the pressure drop from exterior to interior occurs across these seals. Since there is expected to be relatively little airborne water in cavities  182  and  184 , however, any leakage past these seals is likely to be primarily leakage of air. Moreover, since seals  1732  and  1734  are protected from direct sunlight, as well as from mechanical damage, it is expected that these seals will maintain a high level of reliability. 
   A useful method for producing sealing strip  1700  is to feed formed backbone  1600  through an extrusion die so as to extrude shielding fins  1712  and  1722 , along with pressure seals  1732  and  1734 , cross member  1703 , and centering ribs  1742 ,  1744 ,  1746 , and  1748  onto the support. Since shielding fins  1712  and  1722  present visible surfaces when installed, it is useful for them to have a color that is compatible with the units being sealed. Likewise, since base area  1601  between fins  1712  and  1722  is also visible, it is also useful to cover it with extruded material of a similarly suitable color. 
   After extrusion of the polymeric material onto support  1600 , the resulting extruded stock material is cut to length. The length of vertical mull sealing strips is typically less than the height of the window by an amount sufficient to allow insertion of a compressed end seal at each end, while still maintaining compression of the end seal. 
   While sealing strip  1700  has been found effective for sealing vertical gaps in compound fenestration units, an alternative sealing strip, comprising a low shrink backbone portion and conformable sealing portions, along with a drain ramp and drip edge, has been found especially effective for sealing horizontal gaps, while also helping to divert water away from areas of possible leakage. Referring to  FIG. 19 , sealing strip backbone portion  1900  comprises first vertical wall  1901 , from which extend top leg portion  1902  and bottom leg portion  1904  in a first, interior, direction, and from which extends drain ramp  1922  in a second, exterior, direction. Additionally, top anchoring hook portion  1905  is attached to distal edge  1908  of top leg portion  1902 , and bottom anchoring hook portion  1906  is attached to distal edge  1909  of bottom leg portion  1904 . A second, lower, wall  1924  is attached to drain ramp  1922  at its distal edge  1920 . Wall  1924  terminates at drip edge  1926 . A series of drain holes  1930  and  1932  are also provided to enable diversion of leaked water to a harmless location such as the exterior side of the window unit. 
   Referring to  FIGS. 20 -21 , top shielding fin  2040  and bottom shielding fin  2030  provide shielding against wind and rain, while interior pressure seals  2010  and  2020  provide the primary sealing against leakage due to pressure differentials. Referring to  FIG. 21 , interseal cavities  282  and  284  provide dry quiescent zones that enable collection of any water that may have leaked past shielding fin  2040 . Drain holes  1930  and  1932  allow water to drain to a harmless location, such as the exterior of the structure in which the unit is installed. Drain ramp portion  2050  and vertical wall portion  2052  serve to divert rain or other water to drip edge  2053 , where it can fall to the ground or to other harmless locations. Since surfaces  2050 ,  2051 , and  2052  are visible surfaces, it is useful to also coat these surfaces with a suitably colored polymeric material. As shown in  FIG. 21 , horizontal sealing strip  2000  is used to seal horizontal gap  2115  between upper component unit  2110  and lower component unit  2120 . Kerfs  2112  and  2122  are provided for receiving hooks  1905  and  1906 , so as to assure that sealing strip  2000  is inserted to the proper distance during installation and that it is secured in place after installation. 
   While vertical sealing strips  1700  and horizontal sealing strips  2000  can be effective in sealing gaps, it will be recognized that ends of gaps and junctions of gaps will inevitably occur in compound fenestration units. Moreover, gaps in nailing flanges between component units also occur. Referring to  FIG. 22 , a seal for sealing the ends of gaps, along with gaps in nailing flanges, is portrayed. Seal  2200  is made of a conformable foam material and comprises flange gap sealing portion  2210  and gap filler portion  2220 . Seal  2200  can be produced by any suitable means, such as cutting from a solid block of foam, or by adhering suitably dimensioned strips of foam together, as would be apparent to one skilled in the art. It has been found that seal  2200  is more effective in its sealing function if the surface skinning effect commonly encountered in molding of foams can be avoided, so that the porosity of the foam found in the interior of the part also extends to the surface. A useful polymeric material for the foam is EPDM polymer. In addition, it has been found that lubricating the surface of the foam seal with an inert lubricant such as talc prior to installation is useful in easing installation and enabling the foam to properly seat so as to form an acceptable seal. 
   Referring to  FIG. 23 , seal  2200  is shown installed at the top end of vertical sealing strip  1700 , where it cooperates with the top ends of the pressure seals of strip  1700 , and also fills the gap between nailing flanges  2312  and  2322 . An end cover, portrayed in  FIG. 24 , is installed in cavity  1707  to compress gap filler portion  2220  against gap sealing strip  1700  to form a more secure seal and to cover cavity  1707 , while allowing ventilation of cavity  1707 . 
   Referring again to  FIGS. 23 and 24 , top end cover  2450  is comprised of cover portion  2449  and barbed retainer clip portion  2470 . Cover  2450  is installed by inserting clip portion  2470  into cavity  1707  and pressing down until stop rib  2458  engages surface portion  1750  of strip  1700 , and locator notch  2455  of rib  2457  engages surface portion  1759  of strip  1700 . As a result, end portion  2459  of cover  2450  compresses gap filler portion  2220  of foam seal  2200  against the ends of pressure seals  1732  and  1734 , thus completing the pressure seal at the top end, while still allowing ventilation of the interseal cavities and cavity  1707 , as shown in  FIG. 25   a . Compression of filler portion  2220  is maintained by engagement of barbs  2471  with the interior surfaces of cavity  1707  of sealing strip  1700 . Referring again to  FIG. 25   a , it will be noted that the width of cover  2450  is slightly less than the spacing between component units  2310  and  2320 , so as to leave gaps between sealing faces  2501  and  2502  and cover  2450 , which allow ventilation of cavity  1707 , and of interseal cavities  182  and  184 . 
   Referring to  FIG. 25   b , cover  2450  can also be used, along with foam seal  2200 , at the bottom end of vertical sealing strip  1700 . Since cover  2450  is slightly narrower than the gap between sealing faces  2501  and  2502 , drain openings  2510  and  2520  are created, which allow drainage of leaked water from cavity  1707  and interseal cavities  182  and  184 . 
   Referring to  FIG. 26 , the structural strength of the compound fenestration unit can be further enhanced by providing gusset plates such as plate  2600 . Plate  2600  can be made from stamped and bent sheet metal, such as steel. Tabs  2607  engage channels  2605  and  2606  in component units  2310  and  2320 , respectively, to position plate  2600  relative to component units  2310  and  2320 , as well as to position units  2310  and  2320  relative to one another. Additionally, tabs  2607  strengthen the mechanical coupling of plate  2600  to component units  2310  and  2320 . Backup tab  2620  reinforces the attachment of the compound unit to the building structure, and also sandwiches flange gap sealing portion  2210  so as to enhance sealing at the gap between nailing flanges  2312  and  2322 . It will be appreciated that when the compound fenestration unit is installed in a rough opening, screws inserted through holes  2315  serve to not only hold the compound unit in place, but also serve to compress portion  2210  of seal  220  for improved sealing reliability. 
   Junctions of horizontal gaps and vertical gaps, such as junction  9  in  FIG. 1 , also require sealing. An embodiment of a junction seal is portrayed in  FIGS. 27   a  and  27   b . Junction seal  2700  comprises a conformable sealing portion  2710  that is attached to support portion  2720 . A suitable conformable material is polymeric foam, made, for example, by foaming EPDM polymer. Support portion  2720  comprises backbone portion  2723 , which connects front trim portion  2724  with rear base portion  2722 , to which is attached anchoring tab  2725 . It has been found that the sealing effectiveness of conformable portion  2710  can be enhanced by certain shape features. In particular, edges  2712  and  2714  are made as thin as possible, to provide a smooth transition with the sealing face of the fenestration unit, thereby allowing other sealing surfaces, such as pressure seal  2020  of sealing strip  2000  and shielding seal  2030  to fit over them without voids in the sealing area. In addition, the trapezoidal shape of backbone portion  2723  allows conformable portion  2710  to conform to it and thus provide a relatively seamless, void free, transition between surface  2715  of conformable material  2710  and surface  2725  of backbone  2723 . It will be appreciated that cross sectional shapes other than trapezoidal for backbone  2723  may also be suitable, provided that they promote a smooth and void-free conformance of material  2710  to the backbone. 
   Referring to  FIG. 28 , the sealing of gaps at junctions is performed by first installing vertical sealing strips  2815  and  2816 . Top and bottom junction seals  2700  are then installed, as shown, with surface  2712  of conformable material  2710  pressing against the ends of vertical seals  2815  and  2816 , thereby completing the pressure seal. The thin edges of compliant sealing material  2710  form a low profile surface that merges with top surface  2807  of component unit  2806  and top surface  2809  of component unit  2808  to form a sufficiently smooth surface for bottom pressure seal  2020 , shown in  FIG. 20 , of horizontal sealing strip  2000  to seal against it. Finally, referring to  FIG. 29 , horizontal sealing strip  2000  is installed. Because junction seal  2700  is adapted to provide a smooth, void free surface, without sharp transitions, against which interior seals  2010  and  2020  of horizontal sealing strip  2000  can seal, the presence of junction seals  2700  does not significantly disrupt the sealing of strip  2000  against the component window units. Conformable material  2710  is compressed against vertical sealing strips  1700  by pressure seals  2010  and  2020  of horizontal sealing strip  2000 . 
   The ends of horizontal sealing strip  2000  can be sealed by a sealing system of the type shown in  FIG. 30 . Right hand end cap  3050  comprises cover portion  3049  and retainer clip portion  3070 . Cover portion  3049  further comprises notched rib  3057 , wherein notch  3055  fits over the end of wall  3059  and seats against it. Barbed leaf retainer clip  3070  comprises an upper leaf, visible, and a lower leaf, not visible, which fit into cavity  2007 , with barbs  3071  of the upper leaf and lower leaf engaging the upper and lower walls of cavity  2007 , respectively. End cap  3050  is further located relative to end  3051  of strip  2000  by stop rib  3058 , which rests against end  3051 . Surface profile  3054  is shaped to match the exterior profile of the window frame against which it fits, so as to provide a harmonious appearance. In like manner, surface profile  3052  is similar to combined portions  2050  and  2052  of strip  2000 , which it slightly overlaps, as shown by dashed lines  3053  in  FIG. 30 . 
   End seal portion  2220  of flange seal  2200  is interposed between end cap  3050  and end  3051  of strip  2000  so as to provide a pressure seal of cavity  2005 . Barbed retainer clip  3070  is useful in holding end cap  3050  tightly against seal  2220  so as to maintain a level of compression that is adequate for a pressure seal. As shown in  FIGS. 31 and 32 , portion  2210  of flange seal  2200  fits behind nailing flanges  3103  and  3105  of component units  3102  and  3104 , with portion  2220  protruding through the gap between the window units and compressed against sealing strip  2000  by end cap  3050 . Since exterior cavity  2007  is exterior to the pressure seal, it is not necessary for it to be sealed to end cap  3050 , and it is useful for it not to be sealed, so as to provide ventilation to cavity  2007 . 
   Fenestration units of the present invention can be further protected against intrusion of water by the addition of a top drip cap. Referring to  FIGS. 33 , top rail portion  340  of a fenestration unit is shown installed in a rough opening having header  346 , to which is attached sheathing  348 . In this embodiment, top rail portion  340  comprises laminated wood core  342  and cladding  343 . Nailing flange  347  is an integral extension of cladding  343 . Cladding  343  is typically PVC, with an outer cap stock to impart weatherability and improved color, but may also be aluminum or other suitably durable and weatherable material. 
   In a preferred embodiment, drip cap  330  comprises a mechanically and thermally stable core  332 , over which is applied one or more polymeric layers to form nose portion  334 , which is held in a spaced apart position from top rail  340  by spacer rib  335 , so as to move dripping water away from the fenestration unit. Core  332  is preferably a relatively rigid material having a low coefficient of thermal expansion, having a low long term shrinkage. A material that has been found suitable is aluminum, although other materials such as pultruded fiberglass reinforced polymeric materials may also be useful in some applications. An aluminum core may have an advantage in some instances in that it is relatively easy to produce from sheet stock. A preferred polymeric material for the coating layers is PVC, which may be covered with a capstock material such as pigmented PVC or acrylic polymer. Acrylic polymers may be preferable in some instances, depending on color requirements and weathering conditions, for example. Drip cap  330  further comprises flexible sealing flap portion  338  which folds upward and fits against nailing flange  347 . Referring to  FIG. 34 , drip cap  330  is formed by extrusion of polymeric material over aluminum core  332 , with sealing flap extending in a substantially parallel direction with core  332 . Since flap portion  338  is flexible, it can be bent at any suitable point to conform to a variety of fenestration unit dimensions. 
   Referring again to  FIG. 34 , drip cap  330  can be produced as a stock material by extruding polymeric material over core  332 . It is preferred that the extruded polymeric material completely enclose core  332 , and that it impart a suitable color to the visible portion of the drip cap. The polymeric material also forms nose portion  334 , sealing flap  338 , and sonic welding energy directors  331 . When cladding  343  is a sonic weldable material such as PVC, drip cap  330  can be sonically welded to the cladding of the component units. When cladding  343  is not sonically weldable, a dual sided pressure sensitive tape foam tape, or other suitable adhesive means, can be used to attach drip cap  330  to cladding  343 . Because sealing flap  338  is flexible, it can be pulled away from nailing flange  347  during installation to allow fasteners  349  to be installed through nailing flange  347  into sheathing  348  and header  346 , so as to avoid puncturing it, thereby further reducing the risk of leakage.

Summary:
A system is provided for coupling individual fenestration units together and sealing the gaps between them so as to form a sealed compound fenestration unit. The individual fenestration units include mating channels and tabs, or other connectors, that are attached to outside frame surfaces of the individual fenestration units. When the connectors are coupled together, they attach the frames of the individual fenestration units securely together in such a way that gaps are formed between the units along their joined frame surfaces. The gaps are sealed by resilient sealing strips that are configured to be inserted into the gaps, and that are formed with seals that bear against the walls of the gaps to form impervious moisture seals.