fig1 - 2 illustrate a conventional pick - up truck 10 having a cargo box 11 that has a pair of sidewalls 12 and 14 , a front or cab wall 15 , and a rear wall , or as it is more commonly called , a tailgate 16 . these four walls define the perimeter of a cargo box area of the vehicle . a vinyl fabric cover 18 is mounted on the walls to protect the interior of the cargo box 11 as well as its contents . the cover is preferably formed of a flexible fabric , such as a vinyl material used on the outer surfaces of automobiles , which has greater elasticity from side - to - side in the direction of a transverse arrow 19 than it does in the fore and aft position of an arrow 21 . extruded aluminum rails 20 , 22 , 24 , and 25 are mounted along the top of sidewall 12 , tailgate 16 , sidewall 14 and front wall 15 , respectively . the cross - section of the rails 20 and 24 are identical , and the tailgate rail 22 is similar to the side rails 20 and 24 . one embodiment of rail 20 is shown in fig4 for illustrative purposes . referring to fig4 the cargo box sidewalls , such as sidewall 12 , is formed of sheet metal with a horizontal wall 26 and a downward depending , vertical wall 28 . rail 20 has a horizontal section 30 mounted on horizontal wall 26 , and a vertical section 32 which is mounted adjacent vertical wall 28 . liner 33 of a high density polyethylene material is mounted between the rail and the sidewall to prevent squeaks and scratches . a plurality of clamping means identical to clamp 34 and fastener 36 , are mounted along the sidewalls and the tailgate for fastening the rails to the cargo box walls . clamp 34 has a somewhat &# 34 ; c &# 34 ; shaped configuration with a slot 38 receiving both the lower edge of wall 28 and vertical section 32 of the rail . the width of the slot is greater than the combined thickness of the sidewall and the rail . the vertical wall of the rail has a pair of lips 32a and 32b for locating the clamp , and another lip 32c for locating the lower edge of liner 33 . fastener 36 is received through a threaded opening 40 in the clamp to securely attach both the clamp and the rail to the sidewall . fastener 36 is preferably a cadmium plated fastener with rust resistant properties . this clamping arrangement eliminates the necessity for drilling any holes into the cargo box sheet metal . each fastener is concealed behind the sidewalls so as not to scratch the contents of the cargo box . in addition , some trucks have an outwardly directed flange that extends upwardly and outwardly from the top of the sidewalls of the cargo box , thus serving the purpose of the rail 20 . the side of each clamp has a slot 42 for receiving the end of a slat 44 preferably made of wood . slat 44 is one of several , each mounted with their ends mounted in the slots 42 of the clamps 34 to support the midsection of cover 18 . the outer edge of the rail has a lip 48 which extends away from the cargo box area . a similar lip is mounted on the companion rail 24 on the opposite side of the cargo box which also faces away from the cargo box . lip 48 is defined by an upper cam surface 50 and a lower cam surface 52 which intersect to define a pointed edge . the lower edge of cam surface 52 terminates in ridge 54 and slot 56 . the entire lip area is raised above the body of horizontal wall 30 . a &# 34 ; j &# 34 ; shaped , extruded fastener 60 of a polyethylene plastic is stitched at 62 to the edge of cover 18 . the cover is also stitched at 64 to a plastic , stiffening rod 65 . fastener 60 has a curved lower lip 66 forming a partially cylindrical bight 68 for receiving rail lip 48 . lip 66 is resilient such that the extreme edge 70 of the fastener can be moved toward or away from the bight and the lip can unwind like a clock spring . in its relaxed position , edge 70 forms an opening for receiving rail lip 48 . fig5 and 6 illustrate how fastener 60 is connected to the rail by mounting lip 66 on the rail and then pressing downwardly . as the extreme edge of lip 66 engages cam surface 50 of the rail , lip 66 is cammed inwardly until it passes the extreme edge of the rail and then pops outwardly as illustrated in fig6 . a tensile force on the cover in the direction 72 , as illustrated in fig8 will then cause rail lip 48 to be received into the bight of curved lip 66 until the extreme edge 70 of the curved lip slides over ridge 54 into slot 56 . lip 66 is trapped in slot 56 until the user deliberately makes a separating motion to release the cover from the sidewall . fig7 illustrates cover 18 somewhat slack such that there is a loose fit between rail lip 48 and resilient lip 66 . however , in fig8 the application of a tensile force on the cover will then cause the curved lip to essentially unwind in such a manner that , although it remains engaged in slot 56 , the cover can move a distance &# 34 ; a &# 34 ; while being continuously in a taut condition . this unwinding feature permits the fastening structure to accommodate the stretching qualities of the vinyl cover as well as variations in the distance between the sidewalls of pick - up trucks , where the dimensional tolerances may vary as much as a 1 / 4 inch . further the resilient extruded lip accommodates changes in the flexibility of the cover that occur in response to temperature changes . for example , in extremely cold weather , the cover tends to become stiff whereas in warm weather it tends to become very loose . thus , the preferred fastening structure eliminates wrinkles in the cover because it always applies a tensile force to the cover . an identical &# 34 ; j &# 34 ; shaped fastener is connected to the opposite sidewall to cooperate with rail 24 in maintaining the cover in a taut condition . referring to fig4 an extruded plastic molding 74 is adhesively attached to horizontal section 30 of rail 20 . referring to fig3 cover 18 is illustrated partially separated or unzipped from rails 22 and 24 . the &# 34 ; j &# 34 ; shaped fastener has the flexible curved lip trimmed away a couple of inches at 80 so that user 82 can grasp the cover corner and progressively slidably unfasten or unzip the cover from the top rail by an upward motion that unwinds curved section 66 so that the extreme edge 70 of the lip clears ridge 54 and is separated from the rail . fig9 a shows a modified embodiment of a rail 76 suitable for the same purpose as the rail 20 in fig4 . the rail 76 includes a vertical flange 77 that could be attached to the depending wall 28 by means of the clamp shown in fig4 . however , the flange 77 in fig8 is attached to the depending wall by machine screws like screw 83 that not only rigidly attaches the flange to the depending wall , but also passes through a lobe 84 on one side of a saddle portion 85 . the saddle portion is actually located between two lobes , but only lobe 84 is shown in the drawing . in this embodiment the saddle portion receives and supports one end of an aluminum bow 86 that takes the place of the wooden slat 44 shown in fig4 . the top surface of the rail 76 has two flat sections 88 and 90 separated by a valley 92 that can be used as a channel to collect any water that inadvertently works its way under the &# 34 ; j &# 34 ; shaped fastener 60 . since this fastener extends longitudinally along almost the entire sidewall 12 , the only place that water can enter is in limited open areas at the ends of the rail where the rail 76 joins another rail from the next adjacent wall of the cargo box 11 . the part of the extrusion that forms the valley 92 can also serve to space the flat sections 88 and 90 above and away from the upper most part of the sidewall 12 . however , it is not necessary that the bottom surface of the valley 92 even touch the upper edge surface of the sidewall 12 , although it may do so . the part 92 may be considered a base portion of the rail 76 , and the part 90 may be considered to be an outer flange of the rail , which extends upwardly and outwardly away from the base portion , and , therefore , away from the cargo area . the outermost edge 94 of the flange 90 in this embodiment is shown as having a relatively small radius of curvature as viewed in this cross - sectional drawing , and the surface 96 below the edge extends downwardly and inwardly to help define a bead 98 , which is the thickest part of the flange . from the bead , the lower surface 100 curves upwardly and inwardly and then downwardly to define a surface that corresponds to the slot 56 illustrated in fig5 - 8 . although the bight 68 causes the lip 66 of the resilient fastener means 62 to embrace the outer part of the flange 90 , the inner surface of the bight does not necessarily conform closely to the surface of the flange but , as in this instance , touches it only in limited areas . this makes it easier for the bight to uncoil when force from any source pulls the edge part of the cover 18 and the fastener 60 attached to it in the direction of an arrow 102 , as shown in fig9 b . as previously stated , such force can be due to a reduction in the temperature of the environment , causing the cover 18 to shrink , or it can be caused by variations in the manufacturing tolerance of the cover or the cargo box 11 . normally , the extreme edge 70 of the lip 66 is relatively close to or even in contact with the lower surface 100 of the flange as that extreme edge extends inwardly in the general direction of the central part of the cover 18 . however , it is not necessary that there actually be contact between the extreme edge 70 and the surface 100 . when force is applied in the direction of the arrow 102 to force the edge 104 of the main or base portion of the fastener 60 by the distance a to the position shown in fig9 b , the bight 68 uncoils to the necessary extent from its normal curvature , and the extreme edge 70 draws away from the surface 100 . the inner surface of the bight 68 remains in contact with the edge 94 of the flange 90 so that there is no loss of sufficient engagement between the flange and the bight . the flexible cover 18 remains held so that it is still taut across the otherwise open top of the cargo box 11 . fig1 shows a perspective view of only a small fragment of the cargo box 11 , specifically , a fragment near the left front corner of the box at the intersection of the sidewall 12 and the front wall 15 . the rail 76 extends longitudinally along the sidewall 12 to its end 106 . similarly , the rail 25 extends longitudinally along the end wall 15 to the end 107 which leaves a small gap 108 between the ends 106 and 107 . the rail 25 has two outwardly extending flanges 109 and 111 that are parallel to each other and extend longitudinally along the entire length of the rail . these flanges extend upwardly from a base portion 112 that rests on or close to the upper edge of the wall 15 , and the forward edge of the base portion includes a depending flange 113 that extends down along the front surface of the front wall 15 . the extruded fastener 60 attached to the front edge of the cover 18 can be fitted over either of the flanges 109 and 111 depending , primarily , on the season of the year . the material of which the cover 18 is made is more elastic in one direction than it is in the perpendicular direction , and it is preferable to arrange the cover so that it is less elastic in the longitudinal direction thereof , as indicated by the arrow 21 in fig1 than it is in the transverse direction indicated by the arrow 19 . these arrows are also shown in fig1 . arranging the cover 18 so that its greater elasticity is in the transverse direction of the arrow 19 makes it possible to use rails having only a single flange for each rail as in the case of the rails 12 and 14 shown in fig2 and the rail 76 shown in fig9 and 10 . the fastening means 60 along the sides of the cover 18 can be snapped into place on these rails on the sidewalls , even if the cover 18 and the lips 66 have to be stretched to some extent to do so . such stretching of the cover is not possible in the longitudinal direction of the cover 18 as designated by the arrow 21 , and the lips 66 cannot be stretched far enough to do the job , alone , especially if the cover has shrunk due to cold weather . some degree of expansion is possible due to the resilience of the fastening means that engages the rail 22 along the tailgate 16 , but there is not enough resilience to accommodate all of the necessary changes while still keeping the cover 18 taut . by providing two flanges 109 and 111 , the fastening 60 along the front edge of the cover can be made to engage whichever of these flanges produces the best fit . in the winter , it is likely to be the flange 109 and in the summer it is likely to be the flange 111 . fig1 also shows two reinforcing straps 114 and 116 . these are sewn onto end portions of the resilient fastening means 60 along the side edge and forward edge of the cover , respectively , to prevent the cover material from being pulled away from the extruded , resilient fasteners 60 at this corner of the cover . similar reinforcing straps are preferably sewn on at each of the four corners of the cover . fig1 shows the multi - lip rail 25 of fig1 in greater detail . the depending flange 113 is firmly attached to the outer , or forward , surface of the front , or cab , wall 15 by a double - sided , high - density foam tape 118 of a type commonly used to attach panels to vehicles . such tape is used because of its ability to hold metal panels together firmly in all kinds of weather . the pressure exerted on the rail 25 in keeping the cover 18 taut pulls the flange 113 against the tape 118 and the tape against the wall 15 , keeping these components together rather than tending to separate them . the lips 124 and 126 of flanges 109 and 111 have round beads 120 and 122 , and the surfaces 128 and 130 of both flanges extend upwardly from the base 113 and then outwardly , or away , from the central region of the cargo box 11 , forming barriers that have inverted l - shaped cross sections . as in the structure shown in fig7 - 9 , the lip 66 of the fastener 60 curves around either of the lips 124 and 126 and back generally toward the central region of the cover 18 . fig1 shows another embodiment of a rail 132 having multiple lips . the rail 132 has a base 134 and a depending flange that is attached to the front wall 15a of a slightly different cross - sectional configuration than the wall 15 in fig1 . the rail 132 has several barriers , or flanges , 136 - 141 that divide the space above the base 134 into five slots 143 - 147 , each shaped like an inverted t . instead of a j - shaped resilient fastener like the fastener 60 in fig1 , the fastener 148 in fig1 has a forward edge , or flange 149 , that extends down from a base 150 and terminates in a head or cross member , 152 , so that the cross section of the flange 149 with the cross member 152 is an inverted t . the base 150 is stitched or otherwise firmly attached to the cover 18 and it anchors the cover firmly to the rail 132 . the vertical flange 149 of the fastener 148 is narrow enough to pass between the juxtaposed cross members 138 and 139 or between the cross members at the tops of any of the other barriers . while the j - shaped resilient fastener 60 in fig1 can be snapped over at least the flange 111 , and possibly over the flange 109 , the fastener 150 in fig1 cannot be snapped into place that way . in fact , in all multi - barrier rails , it is preferable to slide the fastener in longitudinally into the proper slot . there is ample room to do so , since the thickness of the lip 66 of the j - shaped fastener 60 in fig1 is less than the minimum distance between the flange 109 and the flange 111 . likewise , the thickness of the flange 149 in fig1 is less than the minimum distance between the cross members 154 and 156 . the cross member 152 on the fastener 150 locks under the cross members 154 and 156 on each side of it . as a result , there is contact between the top surfaces 158 and 160 of the cross member 152 and the bottom surfaces 162 and 164 of the cross members 138 and 139 to hold the fastener 150 firmly in place . the same holding structure is available if the cross member 152 is slid into any of the other slots 143 , 144 , 146 , or 147 . in order to describe the sequence of attaching the cover 18 to the cargo box 11 whichever multi - barrier rail is used , reference will be made again to fig2 . first , the fastener means 60 or 150 is slid lengthwise into the slot in the front rail 25 to engage the proper barrier or the barrier believed to be proper . when that edge of the cover is in place , the fastener 60 along either side of the cover is engaged with the rail on that side of the cargo box 11 . third , the resilient fastener 11 at the tailgate 16 is snapped down into engagement with the flange on the tailgate rail . if this is impossible or leaves the cover too loose , the fastener means along the side is released and the front edge of the cover 18 is slid into engagement with a more correct flange on the rail 25 . then the side fastener and rail are re - connected , the fastener 60 and tailgate rail 22 are snapped together , and , finally , the fastener on the other side of the cover is snapped down onto that rail .