Ron

Ron is winning by claiming the tile that completes your hand from another player's discard rather than drawing it yourself. The moment an opponent throws the exact tile you are waiting on, you may declare ron, reveal your hand, and the discarder is said to have 'dealt in' or 'fed' the winning tile. The term ron comes from Japanese riichi, but the same win-by-discard mechanic exists in Chinese, Hong Kong, and Taiwanese mahjong under different names.

The defining feature of ron is who pays. In riichi the single player who discarded the winning tile pays the entire hand value alone, which makes dealing in costly and makes defense (reading what opponents wait on and discarding only safe tiles) a core skill. In many Hong Kong and Chinese house rules the discarder likewise pays the full amount, sometimes with the other players contributing a smaller share depending on the ruleset.

Several rules restrict ron. You cannot ron on a tile you could have claimed earlier and chose to pass on, nor on a tile of the same wait you previously passed, under the furiten rule in riichi: if the winning tile is already in your own discard pile, you are locked out of winning by ron and may only win by self-draw. A closed hand winning by ron also still requires a yaku in riichi, since the win itself grants no yaku the way self-draw can via menzen tsumo.

For example, if you are in tenpai waiting on 3s or 6s and the player across the table discards 6s, you call ron, complete your hand, and that player alone settles your score, assuming none of your discards already contain 3s or 6s, which would put you in furiten.