legs of each triangular locking structure and depend from a basewall section 50 3 at approximately a 60 degree interior angle the basewall sections 50 3 is molded flush with the corner edges 21 a 21 b of the central region 10 a thereby completing the cavity 60 enclosure each male lug 40 fig3 a is disposed substantially equal distances from its laterally opposed outer sidewalls of sections 50 1 and substantially the same distances from their respective tile edges 22 a 22 b thus each male lug 40 is surrounded on three sides by a corresponding female channel 42 of slightly greater width than the width of laterally opposed rib wall sections 50 1 so as to tightly mate with similar but inverted rib wall sections of a contiguous tile the inner sidewalls 50 2 of each rib 50 are also shaped in plan view as an equilateral triangle having rounded interior corners so as to have a substantially identical size and shape as a corresponding inverted lug 40 the resulting open ended cavities 60 fig3 a bottoming on their respective enclosed areas of the surfaces 20 a 20 b have just slightly larger mating interiors than the lug 40 so as to.