8.1 Naka-suji

A 4, 5, or 6 is the wait of two different ryanmen. 5-man is waited on by both a 34-man ryanmen and a 67-man ryanmen.

Toimen has discarded both 2-man and 8-man.

So both ryanmen are in furiten. A tile whose suji on both sides are genbutsu like this is called naka-suji.

For 5-pin, only 2-pin is genbutsu. The 34-pin ryanmen is blocked, but the 67-pin ryanmen remains. A tile whose suji is genbutsu on one side only like this is called kata-suji.

A kata-suji is far more dangerous than a naka-suji. 5-man has both ryanmen blocked, but 5-pin still has the 67-pin ryanmen.

A 4 is naka-suji only when 1 and 7 are both genbutsu. A 5 needs 2 and 8, and a 6 needs 3 and 9.

If only one of 1 and 7 is genbutsu, the 4 is kata-suji. You need to check that both are genbutsu.

Discard one naka-suji tile against toimen.

Discard one naka-suji tile against toimen.

Discard one naka-suji tile against toimen.

Discard one naka-suji tile against toimen.

Discard one naka-suji tile against toimen.