Riichi Mahjong · Yakuman（closed only）

Kokushi Musou (Thirteen Orphans) in Riichi Mahjong: Yakuman Rules, 13-Sided Wait and Odds

In riichi mahjong, Kokushi musou (thirteen orphans) is a yakuman: one of each of the 13 terminals and honors — 1 and 9 of characters, dots and bamboo, the four winds and the three dragons — plus a second copy of any one of them as the pair. It's a special shape outside "four sets and a pair", and you can't call for it.

The rules for Kokushi musou

• Every one of the 13 terminals and honors (1s and 9s of each suit, seven honors)
• One of them doubled as the pair: 14 tiles in all
• Closed only: no chi, pon or kan
• No other yaku needed — it's a yakuman on its own, and isn't added to regular yaku

Two waits: single and 13-sided

Single wait: one kind missing

Twelve kinds once each, one already paired, waiting on the one missing kind.

13-sided wait: one of each

One of each of the 13 kinds and no pair yet: any terminal or honor wins — the 13-sided wait. By default this game follows Mahjong Soul and scores the 13-sided kokushi as a double yakuman; with "Double yakuman" turned off in the lobby, or on Tenhou and in M-League, it's a single yakuman.

Special rules for Kokushi musou

• Chankan: kokushi can rob another player's added kan (any yaku can). Tenhou and Mahjong Soul also let kokushi rob a closed kan; this game currently doesn't — added kans only.
• Nine terminals (kyuushu kyuuhai): if your start plus first draw has nine or more different terminals and honors and nobody has called yet, you may abort the hand. That's exactly when kokushi is closest — nine kinds without a pair is 4-shanten (3-shanten with a pair) — so whether to abort or go for it depends on the score situation.
• No calls: a single call ends kokushi.

How it relates to other yaku

• Chiitoitsu: the other special shape outside "four sets and a pair", also closed only.
• Honroutou: also only terminals and honors, but as four triplets (or seven pairs) and a pair — not a yakuman.
• Chanta: every set contains a terminal or honor; like kokushi it keeps 1s, 9s and honors, but it's a completely different hand.

Kokushi musou odds

Kokushi is almost entirely about how many kinds of terminals and honors you start with. In 1,000,000 random 13-tile deals, a start had 9 or more kinds about 0.15% of the time, 10 or more 0.010%, and 11 or more only 0.0002%.

Source: 1,000,000 random 13-tile deals.

In 40,000 hands between four normal-level computers on this site, kokushi (including the 13-sided wait) was won 13 times; all yakuman together came up 15 times in the same games. Kokushi lives on the deal: the computers only take this road when the start is already full of terminals and honors.

Kokushi musou in other mahjong styles

• Hong Kong mahjong: "Thirteen Orphans" is a limit hand (Hong Kong mahjong fan list)
• Chinese Official (Guobiao): "Thirteen Orphans", 88 fan (Guobiao fan list)

To try it yourself, open a table of free online riichi mahjong and give it a go when a start full of terminals arrives.

FAQ

Is Kokushi musou a yakuman?

Yes. Kokushi musou is a yakuman; the 13-sided wait is a double yakuman by default here (you can turn that off in the lobby; Tenhou and M-League count it as single).

Can you call for Kokushi musou?

No. It's closed only — any chi, pon or kan breaks it.

Can Kokushi musou rob a closed kan?

Not in this game — only added kans. Tenhou and Mahjong Soul allow kokushi to rob a closed kan, so this is a rule difference.

Abort on nine terminals or go for Kokushi?

It depends. Nine kinds with no pair is 4-shanten: you still need four more kinds and a pair, so the success rate isn't high. Unless you're far behind and need a big hand, aborting is a perfectly reasonable choice.

Related yaku

Chiitoitsu 2 han（closed only） Seven different pairs, closed only, 2 han 25 fu. Examples, picking your wait, combos, and how many pairs make it worth it. Honroutou Closed 2 / open 2 Only 1s, 9s and honors — 2 han even when open, always with toitoi or chiitoitsu. Examples, combos and odds. Suuankou Yakuman（closed only） Four concealed triplets, closed only — a yakuman. Shanpon needs tsumo, a single wait wins on ron too (double yakuman here), and how many pairs you need.