Atozuke
Atozuke 「後付け」 describes a "yakuless" tenpai which gains yaku upon winning. In other words, the current state of the hand does not produce yaku while tenpai. This typically involves an open hand, but can also apply to closed hands without riichi. It is also called sakizuke 「先付け」.
When a hand is subject to atozuke, it may win if it draws the "higher value" tile. However, it is usually unable to win off the "lesser" tile, since it would lack yaku. Such hands may still win with the lesser tile if it gains a conditional yaku (like haitei or rinshan). Otherwise, it cannot win. Thus, if the lesser tile is drawn, the hand may be subject to furiten.
Atozuke is subject to rule variations. When the atozuke rule is disabled ("atozuke nashi"), you must have a confirmed yaku before winning (and sometimes, before making any call). The more common rule, by far, is to allow atozuke. Rules disabling atozuke are rare, mostly found in specific mahjong parlors.
Examples
Note: as with any "yakuless" hand, a hand subject to atozuke may win from the lesser tile by gaining haitei, houtei, rinshan, or chankan. Closed hands can also win by mentsumo. If a hand in atozuke does not gain one of these conditional yaku, though, it cannot win by the lesser tile.
Yakuhai
This hand uses a shanpon tenpai for two tiles. Winning with the haku gives a yaku through yakuhai, but winning with the 9-pin gives nothing. As this is a closed hand, it can win with mentsumo regardless of winning tile, but it can only ron off the haku (unless it gains a conditional yaku like houtei).
This is the same hand but with an open call. This hand can only win by haku, regardless of tsumo or ron.
Tanyao
Agari:
Tanyao can only be achieved when won with the 6-sou.
Sanshoku
This hand has an apparent yaku of sanshoku doukou. However, if won with the 3-sou, the hand will contain a 11-sou pair and a 123-sou sequence, ruining the yaku. Thus, it only has a yaku when won with the 2-sou.
Sanshoku is hinted at, however, the extended nobetan wait of 1-4-7s in the hand can ruin sanshoku if won from the 1s (it would become 11 - 234 - 567).
Counter examples
In these examples, atozuke is not used. In these cases, the hands may win on any of the waiting tiles. They each have valid yaku already embedded into the hand, regardless of their winning tiles.
The open call on the green dragon instantly gives the hand a yaku via yakuhai.
This hand employs honitsu.
Atozuke disabled
When the atozuke rule is disabled, the hand must have a confirmed yaku before winning. There are two main variations to atozuke disabled:
- The hand must have a specific yaku confirmed before winning. In addition, when calling, it must have a specific yaku confirmed without needing to make another call and without needing the winning tile. (In most cases, this is equivalent to "you must confirm a yaku when you make the first call".)
- The hand must have a yaku no matter which tile it wins on. Any hand that always has a yaku (any yaku) by the end can win.
To repeat, the atozuke rule is most often enabled. When atozuke is enabled, all of the following types of hand can win without issue.
Two different yaku
If this hand wins from 6-sou, it gains iipeikou. If it wins from 9-sou, it gains sanshoku. When the atozuke rule is enabled, this hand can win no matter what. Both winning tiles give a yaku, so it always has a yaku, so it can win regardless of the tile. Likewise, when rule #2 is used, this hand is allowed to win.
But when rule #1 is used, this hand cannot win. Neither iipeikou nor sanshoku are confirmed. Because this hand does not guarantee "iipeikou", "sanshoku", or any other specific yaku in its current state, it cannot win. (Of course, since this is a closed hand, you can just call riichi.)
Note: when a yaku is not guaranteed because it is implied by another yaku, e.g. when a hand can score sanankou or suuankou, atozuke does not apply, even when using rule #1.
This hand would gain a yakuhai triplet if it wins off the haku or chun. However, it does not have any yaku in the current state, and so it cannot win if rule #1 is in effect.
Yaku confirmed post-call
This hand has a confirmed yaku through the triplet of hatsu. However, it did not have any yaku when calling the 789-sou, and 't needed another call to confirm the yaku. Therefore, this hand cannot win when rule #1 is in effect. With rule #2, this hand can win normally.
Despite sanshoku being guaranteed with the winning tile, because the sanshoku was not complete before reaching tenpai, this hand cannot win under rule #1.
When the hand's closed section guarantees a specific yaku, and the winning tile is not related to completing said yaku, the hand is never subject to atozuke. Even under rule #1, it will always be eligible to win.
- In this case, the hand has a closed hatsu triplet, so it can always win. Even if the hatsu triplet was drawn after the 789-sou call, there would be no practical way to prove you didn't have the hatsu triplet before calling. Therefore, "drawing yaku after calling" is allowed so long as you "complete" the yaku before winning.
- For whole hand yaku, like honitsu and chanta, the hand's closed section is considered to have "guaranteed the yaku". Therefore, even if the hand didn't qualify for honitsu at the time of the first call, you can win with an open honitsu-only hand. (For a yaku like chanta, your waits still need to guarantee chanta to not be atozuke.)
Strategy
When atozuke is enabled, players may deliberately enter the state of atozuke, depending on the starting hand. Calling tiles helps develop the hand faster, even if there's a chance you may not win later. For example, with a yakuhai pair, you may want to call early and hope someone discards the needed yakuhai later.
Risk of furiten
- Draw:
Open hands under atozuke run the risk of furiten. If this hand draws the 9-pin, it enters furiten. If this happens, it is possible to change waits, or win by drawing needed tile. Or you can stay tenpai until exhaustive draw to gain the tenpai payment.