Iishanten

Iishanten is the state of the hand, where one tile is needed to achieve tenpai. This is the closest state of a hand to tenpai.

Four types

The four types are based on the possible configurations regarding complete and incomplete tile groups.

Yojouhai

Kanzenke

Atamanai

Atamanai or headless is a shape, where the hand lacks a pair. Ideal shapes would involve four tiles forming two ryanmen each. If either of the ryanmen completes, then the hand becomes tenpai with a tanki. Otherwise, any of the four tiles can pair up.

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.