rib cavity 60 the second set of male female interlocks is formed by a male rib section 50 1 and its adjoining female cavity 60 as will be apparent the rib 50 follows a substantially semisoidal course a substantial length of each support strip 19 a 19 b the rib merges into the central portion 10 adjacent the tile corners and thus the two endmost lugs 40 a 40 b do not have an intervening rib section the lugs 40 a and 40 b are inverted relative to one another and are laterally spaced by a channel section 42 a the sections 42 a are typically designed to be somewhat wider than the intermediate channels 42 to correspond with the greater width of their respective vertically aligned inverted male rib sections 50 a 50 b of greater width this is done to assist an installer in making alignments and the initial engagements between the corners of contiguous tiles by providing wider interlocks for initial meshing typically the end rib sections 50 a and 50 b encircling a respective one of the endmost cavities 61 and 62 are typically about twice as wide as the intermediate ribs 50 because the rib sections 50 a and 50 b are about twice as wide as the intervening rib sections 50 readily mesh with the correspondingly wider channels 42 a and 42 b by the installer aligning and then simply pressing and corner 24 or 34 of tile 10 with its rib sections 50 a and 50 b and cavities 61 and 62 facing downwards into the upwardly facing lugs 40 a 40 b and wider channels 42 a 42 b.