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When I'm in riichi and draw a red 5 from the wall, am I allowed to exchange it with the non-red 5 in my hand?

I would argue it might be allowed, because it does not change the yaku, wait or hand structure in any way.

In case the above is correct, what happens in case there is an (isolated) 678 in my hand? Would I be allowed to discard the 8? Given the above rationale, the answer should be yes. However, this could be very complex in practice if the 678 were not isolated, or if there are were similar sequences in other colors, and I doubt this would be acceptable for the players to keep exact track of.

In comparison: I've learned earlier that a kan is allowed, when there are 3 equal tiles already present.

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    In general, no. There's very little tracking once you're in riichi, with the exception of concealed kans which are easily verifiable after. You take a tile and keep it separated from your hand. You then either discard that tile, form a concealed kan and take a new one, or declare a winning hand. – Samthere Aug 11 '15 at 14:49
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I think most "official" rules will not allow you to do this. (For reference, I looked up the Japanese Mahjong rules from the European Mahjong association here.)

In principle, I think swapping tiles that do not change the wait structure would be allowed, but it is impossible to verify. In your example of swapping the 5 for the 8, all that is visible is the discarded 8. From an outsider perspective, it is possible that you completed a different meld and simply discarded an 8 from 5-6-7-8 in hand.

It also appears that the hidden kan/kong is allowed, but only when the pung is isolated and cannot be interpreted as a different set of melds (e.g., you cannot draw and complete a kong if you have 1-1-1, 2-2-2, 3-3-3, because those tiles could be interpreted as 1-2-3, 1-2-3, 1-2-3).

  • This is not quite correct. Your hunch that it would not be allowed is correct. Once you declare riichi, all your draws must either complete your hand or be discarded. It is customary to keep your draws distinct from your hand for this reason (such as placing the drawn tile sideways on top of your hand). As for the concealed kan, you can only declare the kan if it does not change your waits. In the 111 222 333 example, you could declare the kan if the rest of your hand were 78 EE, but not if it were 45 EE. See the chombo rules here – Teofrostus Apr 7 '16 at 18:17
  • @Teofrostus The rules you cited don't say that a hidden kan that doesn't change waits is allowed, only that a hidden kan that does change waits is penalized. The rules I linked to explicitly discussed when a concealed kong is allowed: "may declare a concealed kong if a tile is drawn that matches a concealed pung, if this does not change the waiting pattern and if the three tiles to be konged can only be interpreted as a pung in the original riichi hand. (In case of three consecutive pungs in the same suit, no kong may be declared, since the tiles can be interpreted as three identical chows)." – Hao Ye Apr 7 '16 at 21:01
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According to the Japanese Wikipedia article on riichi:

打牌の選択(手牌の入れ替え)ができない。つまり、和了する場合と後述の暗槓する場合を除いて、自摸した牌をそのまま捨てなければならない。

Translating, this means "You are no longer allowed to select your discards (switching the drawn tile with a tile in your hand). In other words, save for drawing your winning tile or calling a concealed kan as described below, you must discard each tile you draw."

That means if you draw a red five after calling riichi, it's either your winning tile and you're calling tsumo, you have three other fives of the same type and you're calling a concealed kan, or you're discarding it. You cannot simply switch a red five with a regular five, even if it wouldn't change your waits.

  • Refering to japanese rules seems a bit authorative. The answer isn't wrong (I don't know any rule variant that would allow this), but you should always consider your local rule variants and/or house rules with these things. The can sometimes deliberately contradict the japanese rules. – DJ Pirtu Feb 26 '16 at 10:22
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    It was the best way to get a definitive answer. The wiki article also doesn't reference any rule variants re: how you select your discards (or don't) during riichi and I've found no other rules wikis/sites which suggest that riichi works in any other way except as described here. The main downside of riichi is that you can no longer choose your discards, meaning you may miss dora tiles or deal into other hands. – Enrico B Feb 28 '16 at 5:09
  • I don't actually think you can have a "definitive" answer as far as Mahjon goes. There are just too many rule variants, all equaly valid in their own regions. As such, one should always take care to consider the local variants first and foremost. – DJ Pirtu Feb 28 '16 at 19:19
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    The whole point of stackexchange is to have definitive answers to concrete questions. =/ – Enrico B Mar 4 '16 at 17:34
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Japanese mahjong does not allow swapping tiles into the hand after riichi, as noted by so many.

Apocryphally, the European Mahjong Association did allow this between 2008 and 2012 (changed that year) because their community's comprehension of the rules as played elsewhere was incomplete and lacking, so they made stuff up to bridge the gaps. Now, EMA rules from 2016 onward are functionally the same to what people would consider to be modern Japanese mahjong, with variance only related to protocol and game etiquette, with a natural focus on a competitive style.

As for using Japanese wikipedia or a pro organization or a jansou chain as a source: it depends who, for what, and under what optics. Using a single source might not always work, but if they all conform to one or two clear options, then it does make sense. There are people who have their done their research on this and other topics already. Some things do have variance: this isn't one of them. House rules govern agreements between 4 players that have chosen to abide by them, but this does not make them normalized or "an equivalent option" because they simply are not common or relevant enough to make that distinction.

  • Could you provide a source for EMA allowing this? I have played for many years in Europe, and I was always told that the only three options when picking a tile after calling riichi were: tsumo, kan (with the caveats on not changing the ways the hand could be interpreted), discard. – Tobias Kildetoft Sep 5 '16 at 8:25
  • No, and I will actively refuse to search this for you. Two reasons: – Senjooooo Sep 5 '16 at 9:48
  • (Three. Limited edit time.) * Their source was literally one guy in the Netherlands playing nashi-nashi, around 1999-2000. * Integrating a tile in your hand post-riichi lets players cheat. I personally advocate against both the cheating aspect and the fact that no source we know does this. (Maybe some oddball jansou has allowed this, burden of proof is on them: even if there is, it isn't common enough to justify) * You have the information to find it in an internet archive. There is a reason the rule was changed: because it's a bad rule. Other groups in Europe never allowed this. – Senjooooo Sep 5 '16 at 10:03

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