You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to door bolting devices and, more particularly, to a door bolt adapted to lock the door, enabling the door to be slightly opened without letting people outside the house pass to the inside of the house.  
           [0003]    2. Description of the Related Art  
           [0004]    Various door locking devices have been disclosed for use to lock the door, and have appeared on the market. In order to let the door be slightly opened without allowing people outside the house to pass to the inside of the house, a door bolt may be used. A conventional door bolt is known comprised of doorplate and a jamb plate. The doorplate is fixedly fastened to the free end of the door panel, having an elongated sliding slot. The jamb plate is fixedly fastened to the doorjamb, having a chain and a slide bolt at the end of the chain for coupling to the sliding slot of the doorplate. There is known another design of door bolt in which the jamb plate has a hinged retainer bar; the doorplate has a hook. When the door closed, the hinged retainer bar is turned toward the hook of the doorplate, enabling the longitudinal sliding slot of the hinged retainer bar to be coupled to the hook of the doorplate.  
           [0005]    The aforesaid prior art door bolts are still not satisfactory in function. If the person inside the house meets with an accident and is unable to open the door, any person outside the house cannot unlock the door bolt from the outside. In this case, a person outside the house must destroy the door to get into the inside of the house.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention has been accomplished under the circumstances in view. It is the main object of the present invention to provide a door bolt, which enables a person outside the house to unlock the door bolt with a key. To achieve this and other objects of the present invention, the door bolt comprises a jam plate fixedly fastened to a doorjamb, and a doorplate assembly installed in the free end of a door panel and adapted to secure the free end of the door panel to the jamb plate, for enabling the door panel to be slightly opened without letting people outside the house pass to the inside of the house. The doorplate assembly comprises a casing, a hollow locating frame bar mounted within the casing, a movable frame bar axially slidably inserted into the locating frame bar, two locking straps coupled to the movable frame bar and the locating frame bar, and a lock cylinder controlled by a key to move the movable frame bar in the locating frame bar and to further move the two locking straps relative to each other between a first position where the doorplate assembly is prohibited from escaping out of the constraint of the jamb plate and a second position where the doorplate assembly is released from the constraint of the jamb plate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is an elevational view of a doorplate assembly and a jamb plate for a door bolt according to the present invention.  
         [0008]    [0008]FIG. 2 is an exploded view of the door bolt according to the present invention.  
         [0009]    [0009]FIG. 3 is a front plain view of the present invention showing the locking straps moved to the unlocking position.  
         [0010]    [0010]FIG. 4 is similar to FIG. 3 but showing the locking straps moved to the locking position.  
         [0011]    [0011]FIG. 5 is a side plain view of the present invention showing the doorplate assembly disconnected from the jamb plate.  
         [0012]    [0012]FIG. 6 is another side plain view of the present invention, showing the doorplate assembly coupled to the jamb plate, the locking straps moved to the locking position.  
         [0013]    [0013]FIG. 7 is a schematic drawing, showing the doorplate assembly and the jamb plate respectively installed in the door panel and the doorjamb, the doorplate assembly secured to the jamb plate, the door panel closed.  
         [0014]    [0014]FIG. 8 is a schematic drawing, showing the doorplate assembly secured to the jamb plate and the door panel slightly opened,  
         [0015]    [0015]FIG. 9 is a schematic drawing showing the doorplate assembly disconnected from the jamb plate, the door panel opened. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    Referring to FIG. 1, a door bolt in accordance with the present invention is shown comprised of a doorplate assembly L and a jamb plate  8 . The doorplate assembly L comprises an elongated casing  1 , a mounting plate  6 , and a lock cylinder  7 . The casing  1  is a hollow rectangular member having an opening  10  in one end there. The top and bottom sidewalls of the opening (open end)  10  of the casing  1  are fastened pivotally with the mounting plate  6  by pivot pins  62  so that the casing  1  can be turned relative to the mounting plate  6 . The lock cylinder  7  is provided at the bottom side of the casing  1 . The mounting plate  6  and the jamb plate  8  are respectively fixedly fastened to a door panel A and a doorjamb B (see FIG. 7). The jamb plate  8  has a bolt  81  perpendicularly extended from the front side. The bolt  81  has a head  82 .  
         [0017]    Referring to FIGS. 2 and 3 and FIG. 1 again, the doorplate assembly L is comprised of the aforesaid casing  1 , a locating frame bar  2 , a movable frame bar  3 , a top locking strap  4 , a bottom locking strap  5 , the aforesaid mounting plate  6 , the aforesaid lock cylinder  7 , and a link  73 . The other end (opposite to the open end  10 ) of casing  1  is a close end  11 . The casing  1  has a sliding slot  12  longitudinally extended from the close end  11  to the open end  10  in the front sidewall, a back hole  14  in the back sidewall, two pivot holes  13  in the top and bottom sidewalls corresponding to the open end  10 , and two beveled guide edges  101  formed in the front sidewall corresponding to the open end  10  to expand one end of the sliding slot  12 .  
         [0018]    The locating frame bar  2  is a hollow rectangular frame bar fitted into the casing  1 , having a longitudinal sliding slot  21  extended in the front sidewall between the two distal open ends, a plurality of elongated front holes  22  symmetrically disposed in the front sidewall in vertical direction at two sides of the sliding slot  21 , and a back hole  23  in the back sidewall.  
         [0019]    The movable frame bar  3  is a hollow rectangular frame bar fitted into the locating frame bar  2 , having a longitudinal sliding slot  31  extended in the front sidewall between the two distal open ends, a plurality of first elongated front holes  32  and second elongated front holes  33  symmetrically and obliquely disposed in the front sidewall at different elevations above and below the sliding slot  31 , and a back hole  34  in the back sidewall. The first elongated front holes  32  each have a top end portion  321  and a bottom end portion  322  (see FIG. 3). The second elongated front holes  33  each have a top end portion  331  and a bottom end portion  332  (see FIG. 3).  
         [0020]    The top locking strap  4  and the bottom locking strap  5  each have a plurality of pegs  41  or  51  perpendicularly protruded from the respective back sidewall corresponding to the elongated front holes  22  of the locating frame bar  2 .  
         [0021]    The assembly process of the casing  1 , the locating frame bar  2 , the movable frame bar  3 , the top locking strap  4  and the bottom locking strap  5  is outlined hereinafter with reference to FIGS. 2, 3 and  5 . The movable frame bar  3  is inserted into the locating frame bar  2 , keeping the first and second elongated front holes  32  and  33  respectively aimed at the elongated front holes  22  of the locating frame bar  2 , and then the top locking strap  4  is attached to the front sidewall of the locating frame bar  2  to force the pegs  41  into the respective elongated front holes  22  above the sliding slot  21  of the locating frame bar  2  and the first elongated front holes  32  of the movable frame bar  3 , and then the bottom locking strap  5  is attached to the front sidewall of the locating frame bar  2  to force the pegs  51  into the respective elongated front holes  22  below the sliding slot  21  of the locating frame bar  2  and the second elongated front holes  33  of the movable frame bar  3 , and then the top and bottom locking straps  4  and  5  are inserted with the locating frame bar  2  and the movable frame bar  3  through the open end  10  into the inside of the casing  1 , keeping the back hole  14  of the casing  1  aimed at the back hole  23  of the locating frame bar  2  and the back hole  34  of the movable frame bar  3 .  
         [0022]    Referring to FIGS. 1 and 2 again, the mounting plate  6  has two parallel lugs  61  perpendicularly extended from the front sidewall and spaced from each other at a distance approximately equal to the vertical width of the casing  1 . The lugs  61  each have a pin hole  610 . The aforesaid pivot pins  62  are respectively mounted in the pin holes  610  of the lugs  61  of the mounting plate  6  and respectively inserted into the pivot holes  13  in the top and bottom sidewall of the casing  1  to pivotally secure the open end  10  of the casing  1  to the mounting plate  6 . The lock cylinder  7  has a plurality of mounting lugs  72  respectively fixedly fastened to the back sidewall of the casing  1  by screws. The link  73  has a coupling hole  731  formed in one end and coupled to the spindle (not shown) of the lock cylinder  7 , and a coupling rod  732  extended from the other end and inserted through the back hole  14  of the casing  1  and the back hole  23  of the locating frame bar  2  and fitted into the back hole  34  of the movable frame bar  3 . The back hole  14  of the casing  1  and the back hole  23  of the locating frame bar  2  are longitudinal sliding holes. When the user inserted the key (not shown) into the keyway  71  of the lock cylinder  7  and rotated, the link  73  is driven by the spindle of the lock cylinder  7  to move the movable frame bar  3  in the locating frame bar  2 .  
         [0023]    Referring to FIGS. 4 and 6, when the movable frame bar  73  moved rightwards in the locating frame bar  2 , the first and second elongated front holes  32  and  33  of the movable frame bar  3  guide the pegs  41  and  51  of the top and bottom locking straps  4  and  5  to move to the bottom end portions  322  of the first elongated front holes  32  and the top end portions  331  of the second elongated front holes  33  respectively, and therefore the top locking strap  4  and the bottom locking strap  5  are moved in direction toward each other to narrow the vertical width of the sliding slot  12  of the casing  2 . At this time, the bolt  81  of the jamb plate  8  can be inserted through the open end  10  into the inside of the casing  1  and moved along the sliding slot  12 . Because the vertical width of the sliding slot  21  is narrowed by the top and bottom locking straps  4  and  5 , the top and bottom locking straps  4  and  5  stop the head  82  of the bolt  81  inside the casing  1 , and the bolt  81  is prohibited from transverse movement across the sliding slot  12 .  
         [0024]    Referring to FIGS. 3 and 5 again, when wishing to disconnect the jamb plate  8  from the casing  1 , insert the key into the keyway  71  of the lock cylinder  7  and then rotate the key in the reversed direction to move the movable frame bar  3  leftwards. At this time, the first and second elongated front holes  32  and  33  of the movable frame bar  3  guide the pegs  41  and  51  of the top and bottom locking straps  4  and  5  to move to the top end portions  321  of the first elongated front holes  32  and the bottom end portions  332  of the second elongated front holes  33  respectively, and therefore the top locking strap  4  and the bottom locking strap  5  are moved in direction apart from each other to fully open the sliding slot  12  of the casing  2 , for enabling the head  82  to be moved with bolt  81  of the jamb plate  8  across the sliding slot  12  to the outside of the casing  1 .  
         [0025]    Referring to FIGS.  7 - 9 , the doorplate assembly L and the jamb plate  8  are respectively fastened to the door panel A and the doorjamb B. When the door panel A closed, the doorplate assembly L is coupled to the jamb plate  8 . When opening the door panel A after the doorplate assembly L has been coupled to the jamb plate  8 , the jamb plate  8  constrains the doorplate assembly L, prohibiting the door panel A from being opened (see FIG. 8). A person outside the door can use the key to unlock the lock cylinder of the door plate assembly L and to release the door plate assembly L from the constraint of the jamb plate  8 , enabling the door panel A to be opened (see FIG. 9).  
         [0026]    A prototype of door bolt has been constructed with the features of the annexed drawings of FIGS.  1 - 9 . The door bolt functions smoothly to provide all of the features discussed earlier.  
         [0027]    Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

Summary:
A door bolt is constructed to include a jam plate fixedly fastened to a doorjamb, and a doorplate assembly installed in the free end of a door panel and adapted to secure the free end of the door panel to the jamb plate, the doorplate assembly having a lock cylinder controlled by a key to move a movable frame bar in a fixed locating frame bar and to further move two locking straps relative to each other between a first position where the doorplate assembly is prohibited from escaping out of the constraint of the jamb plate and a second position where the doorplate assembly is released from the constraint of the jamb plate.