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What is the reasoning beyond Kuikae (喰い替え) rule in riichi mahjong?

What exactly are the situations that this rule prevents?

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Kuikae prevents the following:

  • People calling for a cheap defense against an opponent's riichi. If they have discarded a 6 and you have 789 in your hand, without kuikae, you can call the 6 to discard a 9 from an already complete run in your hand.
  • People scaling their hands towards tanyao/chanta by directly discarding the tile on the other end.
  • People scaling their hands towards sanshoku, trying to align runs like 456x, 567y, 678z by calling 7x and 5z to obtain 567x, 567y, 567z.
  • As well as some places (rare, but it does exist) that allow people to call and discard the exact same tile. 5+67, discard 5... or even 5+55, discard 5.

Standard penalty for breaking this rule is a dead hand, in most competitive leagues as well as in jansô environments.

EDIT: For those reading, we talk about kuikae, but in the sense the the rule is a restriction on what can legally be done (making these cases illegal moves), so an OFF setting, as opposed to an ON setting. Kind of like the kô snapback ban in go, or rules relating to repeated positions in chess/shôgi.

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    Out of curiosity, why are these things a problem? They seem like legitimate ways of playing to me.
    – Samthere
    Apr 9 '18 at 15:21
  • If you're playing in jansô establishments for money, then there are often expectations on what moves are allowed or not, and what can give an impression of cheating. Most competitive mahjong environments (no gambling) allow for some of this to varying degrees. In this case, it's less what one player thinks is right, but what the community accepts as right. It's almost always communicated in jansô rules what they tolerate (most are strict kuikae, but a good number will allow anything but the same tile discarded.) Always verify prior to playing in a jansô, a tournament or with strangers
    – Senjooooo
    May 1 '18 at 22:58
  • IIRC, EMA rules allow the first three cases; only the last one is forbidden. Apr 28 '19 at 7:17
  • EMA2008 did allow the above (definitely including the last point in 2011), I can't remember the 2012 revision, but it's all been banned in EMA2016, the most recent rule version (Section 3.3.2)... ah it says in the 2016 revision that it was changed then, probably due to the influence of the World Riichi Championship held in 2014, then in 2017 and (likely 2021) with the WRC rules devised for a clean competition of skill. There's still differences between them, but it's converging closer over time.
    – Senjooooo
    Jun 4 '20 at 4:00

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