fig3 illustrates the basic components of a rail system which will be employed , in accordance with the present invention , in combination with a flexible sheet of material to form a tonneau cover for an opening . the principal use contemplated for a system according to the present invention is as a cover for an open truck bed , as illustrated in fig1 . however , one can imagine other advantageous uses for covering openings of stationary structures , open boat areas , etc . and it is expected that those purchasing such a system may utilize it in a wide variety of imaginative ways . the contemplated system includes , firstly , a frame which may have the form shown in fig3 and is composed of a plurality of side rails 4 , a front rail 6 and a rear rail 8 . each side of the frame system may be defined by one or more side rails 4 and when more than one side rail 4 is disposed along each side of the frame , they may be joined together by suitable rail couplers 10 which will engage rails 4 in a manner to be described below . similarly , although it is the usual practice for each of the front and rear rails to be made in one piece , each of these rails could also be made in more than one piece , with the pieces being connected together by a suitable coupler 10 . in addition , one or both of front rail 6 and rear rail 8 could be replaced by a simple stiffening element to which the flexible sheet would not be attached . for example , in the case of a system for covering a truck bed , it may not be practical to secure the flexible cover to the front rail . in each corner of the frame , front rail 6 and rear rail 8 will be connected to adjacent side rails 4 by suitable corner couplers 12 which will be assembled to rails 4 , 6 and 8 in a manner that will be described below . a cover system according to the invention is intended to cover an opening in a structure and the above - described frame is preferably secured to the sidewalls of such opening by suitable clamps 14 which may have any known configuration . the illustrated configuration of clamps 14 enables them to clamp rails 4 , 6 and 8 to a horizontal surface 48 of a structure which is open toward the interior of the opening to be covered , as will be described in further detail below . if rails 4 , 6 and 8 are to be secured to other sidewall arrangements , such as vertical panels , other known clamp elements can be employed . in any event , rails 4 , 6 and 8 could always be fastened to sidewalls of the structure to be covered by bolting or by means of sheet metal screws or other suitable fasteners . rails 4 , 6 , and 8 may further include a foam tape 60 affixed to a base piece 22 of rail 4 , 6 , and 8 , as illustrated in fig5 and 6 . the structure illustrated in fig3 may be completed by arcuate stays 16 , known in the industry as support bows or tonneau bows . as illustrated in fig1 and 2 , the stays 16 will give a roof - like form to a flexible cover sheet 50 provided to cover the opening . stays 16 are installed in such a manner as to be readily removable when they are not desired . one suitable embodiment of each rail 4 , 6 , 8 is shown in cross section in fig5 and 6 . the illustrated embodiment includes a horizontal base piece 22 , a top flange 24 and a vertical web 26 extending between base piece 22 and top flange 24 . web 26 carries an inclined projection 28 . one end of a stay 16 is to be held in place between projection 28 and base piece 22 . normally , the base piece 22 of front and rear rails 6 and 8 can be made substantially narrower than is illustrated in fig5 and 6 , or may be eliminated entirely . in addition , rails 6 and 8 need not be provided with a projection 28 . affixed to the bottom of base piece 22 is foam tape 60 . the tape 60 acts to cushion the frame and will prevent the vehicle sidewalls from becoming scratched or marked . further , the tape acts as a moisture barrier and a weather stripping seal between the frame system and the sidewalls . in a preferred embodiment , the tape is polyethylene with an adhesive backing . each rail 4 , 6 , 8 further includes an inclined web 29 which cooperates with portions of web 26 and base piece 22 to form a triangular passage 34 . each end of passage 34 can receive one end of a coupler 10 or 12 . one end of base piece 22 is folded up to form a retaining lip 36 and top flange 24 carries a projecting member 40 having a free end which faces toward web 29 . web 29 , lip 36 and projecting member 40 cooperate to form a channel 35 for retaining one or more fasteners 18 . preferred embodiments of the fasteners are shown in fig4 a and 4b and in cooperation with the frame in fig5 and 6 . the fasteners 18 and 18 &# 39 ; have a first substantially flat base portion 20 . extending from base portion 20 are a first leg 32 and a second leg 30 . first leg 32 has an outwardly projecting lip 41 formed on a terminal end . second leg 30 has an outwardly projecting lip 42 formed on a terminal end as illustrated in fig4 b . second leg 30 may also have a second outwardly projecting lip 42b as is illustrated in fig4 a . second lip 42b is shorter than lip 42 . fastener 18 also has a locking leg 31 extending from base portion 20 and inclined towards first leg 32 . when the system according to the present invention is to be assembled on a horizontal wall 48 of a structure presenting an opening to be covered , base pieces 22 of at least rails 4 will be clamped to wall 48 by means of clamps 14 . the opening will then be covered by a flexible sheet 50 of plastic , canvas , or other durable material constituting a tonneau cover , or tarp , cover sheet 50 being secured to fasteners 18 in a manner to be described below . the periphery of cover sheet 50 carries a plurality of fasteners 18 or 18 &# 39 ; each permanently secured to cover sheet 50 , as by stitching , stapling , pinning , etc . one such fastener 18 is shown alone in fig4 a and 4b . also shown in greater detail is a fastener 18 , 18 &# 39 ; connected to a cover sheet 50 and engaged with rail 4 in fig5 and 6 . each fastener 18 , 18 &# 39 ; is preferably made of a plastic material , such as a black acetal plastic , a material which is not subject to corrosion or other forms of deterioration and which can be penetrated in order to be secured to cover sheet 50 in one of the ways mentioned above . turning to fig5 and 6 , it may be seen that one embodiment of a fastener 18 according to the invention includes flat , elongated upper and lower connecting flanges 19 and 21 via which fastener 18 can be attached to cover sheet 50 as by stitching 66 . flanges 19 and 21 permit the transmission of stresses between cover sheet 50 and each fastener 18 to be distributed over a relatively large area of cover sheet 50 . as is illustrated in fig4 - 6 , fastener 18 further includes a hinge section 25 located above first leg 32 , hinge section 25 being formed by providing a radial cutout in base portion 20 . hinge section 25 allows first leg 32 to flex and then engage channel 35 formed by web 30 , retaining lip 36 and projecting member 40 . referencing fig5 attachment of fastener 18 to rail 4 will now be described . to bring a fastener 18 into engagement with the channel 35 , sheet 50 must be stretched over the top of flange 24 . upper connecting flanges 19 is placed in line with the top of projecting member 40 . second leg 30 is pressed against projecting member 40 as fastener 18 is lowered into channel 35 . a downward force is then applied to lower connecting strip 21 . as the force is applied , lip 42 and lip 42b engage projecting member 40 and first leg 32 folds in and slips over retaining lip 36 . as first leg 32 folds in , it engages locking leg 31 . when the applied force is removed from lower connecting flanges 21 , locking leg 31 pushes first leg 32 downward and presses lower leg 32 into engagement with retaining lip 36 such that fastener 18 is securely retained in channel 35 . when fastener 18 is placed into channel 35 , locking leg 31 acts to prevent lower leg 32 and therein fastener 18 from accidentally disengaging from channel 35 . referencing fig6 removal of fastener 18 from rail 4 will now be described . in order to release fastener 18 from channel 35 , one simply applies pressure horizontally to lower connecting flange 21 , towards the center of the covered opening and in the direction of arrow 62 , then lifts lower connecting flange 21 upward in the direction of arrow 63 . as pressure is applied in the horizontal direction , first leg 32 engages locking leg 31 and is lifted upwards away from web 29 . this movement allows first leg 32 and lip 41 to be lifted over retaining lip 36 when an upward force is applied to lower connecting flange 1 in the direction of arrow 63 . the fastening arrangement according to the present invention will successfully resist forces imposed on cover sheet 50 in virtually any other direction . since all fasteners 18 are arranged to face inwardly relative to the covered opening and are inclined relative to the vertical , and cover sheet 50 is stretched over top flange 24 , cover sheet 50 can be placed under tension and a neat connecting arrangement achieved , even if stays 16 are not provided . while the description above refers to particular embodiments of the present invention , it will be understood that many modifications may be made without departing from the spirit thereof . the accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention . the presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims , rather than the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .