Main four types
The four types are based on the possible configurations regarding complete and incomplete tile groups.
Floating tile iishanten 「余剰牌」 describes the case where there are two complete groups and a pair, with two incomplete groups waiting to be completed. One tile (the East tile in the example hand) does not contribute to any group, and so is said to be floating.
Complete iishanten 「完全形、完全イーシャンテン」 is an improvement on floating iishanten when all tiles in the hand contribute to the iishanten. In the example hand, the 223s group now has three tiles working together. This hand is also an example of perfect iishanten. Perfect iishanten is a special type of complete iishanten that guarantees a ryanmen wait. If we draw 1469s, we can discard 2s for a ryanmen wait. If we draw 2s or 2p, we discard 3s to wait on 69s to complete the 78s group.
Headless iishanten 「頭なし、ヘッドレス」 is a hand where we have three complete groups, no pair, and one or two incomplete groups. An ideal headless hand will have two ryanmen groups waiting to be completed. If either of these groups completes, then the hand becomes tenpai with a tanki wait. If we pair one of the four tiles, then we can discard the other tile in its incomplete group to get a ryanmen wait on the other incomplete group.
With an ankou in the hand, the above shape becomes even more advantageous. The completion of either of the ryanmen can bring about pinfu by discarding one tile from the ankou and use the remaining two as the pair.
Sticky iishanten 「くっつき」 is identified by three complete groups, one pair, and two floating tiles (37s in the example above). This type of iishanten tends to have the largest range of acceptable tiles to bring it to tenpai (any souzu tile in the example). However, the downside is that many of these draws may leave a narrow, non-ryanmen wait.
Chiitoitsu achieves 1-shanten, when the developing hand is at five pairs. That potentially leaves three individual tiles waiting to be paired. At least one of the tiles could be occupied as part of a triplet, which actually reduces possible pairing tiles down to two.
- Iishanten in Japanese Wikipedia
- 【麻雀】上がりを増やす！イーシャンテンの秘密【初心者向け講座】 (YouTube)
- Video covering iishanten shapes