Simple

Simples are the number tiles from 2 through 8 in each of the three suits: 2m-8m, 2p-8p, and 2s-8s. They are everything that is neither a terminal (1 or 9) nor an honor (wind or dragon). Simples are the most flexible tiles in the game because every one of them can sit inside multiple sequences, which makes them the backbone of fast, chow-heavy hands.

The key scoring rule attached to simples is tanyao, 'all simples,' a hand containing only 2-through-8 tiles with no terminals or honors anywhere. In Japanese riichi tanyao is a one-han yaku that can be made even with an open hand under most rulesets, making it one of the easiest and most common ways to secure a valid hand. Many Chinese and Hong Kong rule sets give a similar 'all simples' bonus. Because simples never qualify as yakuhai, a triplet of them scores less fu than a triplet of terminals or honors in riichi.

Strategically, a central simple such as 5p is the most connective tile in the game: it can complete 3p-4p, 4p-6p, or 6p-7p, giving it three ways to extend, whereas a terminal extends only one way. This is why discarding middle simples early is risky and why they are usually kept. They are also the tiles most likely to be involved in two-sided waits.

For example, a hand of 234m 567p 678s 345s with a 6m-6m pair is a clean tanyao, since every tile lies between 2 and 8. Swap the pair to 9m-9m and the terminal instantly breaks tanyao, removing the yaku and, in a closed hand with no other yaku, leaving you unable to win at all.