The ryanmen (両面, "two faces") is a wait made of two consecutive tiles of the same suit, open at both ends. For example, 4-5 of bamboo (4s-5s) completes into a sequence (順子) with either the 3s or the 6s. Unlike the kanchan (closed middle wait) or the penchan (edge wait blocked by a terminal), both of its sides are open.
A ryanmen accepts two tile values, up to 8 unseen copies before exposed tiles are taken into account. It is also the only wait shape that satisfies pinfu, which requires the winning tile to complete an open-ended sequence. With value, risk and visible copies held comparable, a ryanmen therefore offers more ukeire than a kanchan.
Choosing your wait shape is a core decision of efficient play. With shanten, value and risk held equal, ryanmen offers wider ukeire and access to pinfu, but dora, yaku and visible copies can change the choice. On defense, ryanmen is the basis of suji: an opponent who discarded 6 cannot ron 3 with a ryanmen because that same shape also waits on 6, which would make them furiten. Always check your own kawa before declaring riichi, since a ryanmen with one end already discarded leaves you in furiten.