a vertical stem portion 16 extending substantially vertically upward from the base strip 11 and a pair of relatively thin hook shaped engaging arm portions 17 bifurcating from a top end of the stem portion 16 in opposite directions longitudinally of the base strip 11 the distribution of the interengaging elements 12 over the base strip 11 is such that the elements 12 are aligned transversely in every other rows and offset in position longitudinally with respect those in adjacent or neighboring rows this arrangement provides engaging spaces 18 in one base strip 11 for receiving the interengaging elements 12 on the other mating base strip 11 39 the length l of one of the two bifurcated arm portions 17 is substantially equal preferably slightly smaller than the distance d between the side line 16a of the stem 16 and the next adjoining row of elements 12 as shown in fig2 additionally as shown in fig4 each one of the two engaging arm portions 17 has its peripheral tip edge 17a disposed slightly apart from the side line 14b of the adjacent rib 14 thereby facilitating the formation by injection molding of the interengaging elements 12 as better shown in fig2 and 4 the bifurcated arms 17 each are arcuately shaped to provide a rounded outer peripheral surface 17b which is smoothly engageable with and easily deformable mutually with a counterpart 17b on the opposite mating base strip 11 39 so as to facilitate coupling and uncoupling of a pair of strips 11 11 39 as illustrated in.