Suji

Suji 「筋」 is the principle of utilizing the "mahjong octaves". This applies to the numbered suits: souzu, pinzu, and manzu. While these three suits are numbered 1 through 9, the numbers may be arranged in a square fashion to determine the "octaves". The purpose is to deduce and/or determine tiles waiting via open waits.

Defense

Defending using suji essentially presumes a player to be utilizing either the wait pattern of ryanmen, ryanmenten, nobetan, sanmentan, or some other variation. Under the rule of furiten, if a player discarded a waiting tile, then the player's ability to call "ron" is disabled. Therefore, certain tile discards may rule out certain mahjong octaves.

Offense

5-pin discarded to indicate 2-pin or 8-pin as safe, when one is not.

The strategy of suji may be used offensively. By utilizing suji in reverse, a player may use a tile within an "octave" to lure out a winning tile along the same "octave". This is commonly known as the "suji" trap.