The principle that only the pattern of suits matters strategically, not which specific suits appear. Rainbow, two-tone, and monotone are the three classes. This reduces 22,100 distinct flops to roughly 1,755 rank-and-pattern classes, and for practical strategy to a few hundred textures. A heart-heavy flop and a spade-heavy flop with the same ranks are the same game. The exception is when hole cards break the symmetry — your own suitedness maps onto one of the board's suits.