Exclusion rule

The exclusion rule (sometimes called the non-repeat or non-identical principle) is the accounting backbone of Chinese Official scoring (MCR). Because MCR defines 81 named patterns and a single hand can satisfy several at once, the rules need a consistent way to decide which patterns may be counted together and which may not, so that the same tiles are not scored twice under different names.

The core idea is that a larger pattern which by definition already contains a smaller one suppresses that smaller pattern — you score the big one and do not separately add the component it implies. For instance, Pure Straight (a 123-456-789 run in one suit) inherently contains shorter run patterns, and All Types or flush patterns subsume lesser suit-based scores. Likewise certain pung-based patterns absorb the points of the lesser pung patterns nested inside them. There are also 'account once' style restrictions where a feature already used to justify one pattern cannot be reused to justify another.

This is why MCR scoring is not simply additive: a beginner who tallies every pattern they can spot will routinely overcount. The official tables and the implication relationships between patterns govern exactly which combinations are permitted. For example, a hand scoring Full Flush already guarantees it is not a mixed-suit hand, so you cannot also claim a half-flush pattern on top. Mastering the exclusion relationships is one of the hardest parts of MCR for newcomers, and tournament scoring software encodes these rules so that each hand resolves to a single correct, maximal but non-redundant total. Hong Kong and Riichi handle overlap differently — Riichi generally adds han with only a few explicit upgrade/exclusion cases — so the exclusion rule is most central to MCR play.