Suji (2)
This page goes one step deeper into suji.
Suji is one of the most important concepts in mahjong, but it is also one of the easiest to misuse.
Distant Suji?
This is a very common beginner misunderstanding.
For example, against the riichi below:
Since has already been discarded, 
and 
are suji.
That means they are at least somewhat safer, because they will not deal into a ryanmen wait.
But even if has been discarded, you still cannot say that 
and 
are safe.
Sometimes you see people call this "distant suji" and cut those tiles, but it does not mean anything.
That said, even among non-suji tiles, there is still a danger difference between 
and .
can hit:
both types of ryanmen.
By contrast, for , one side, the 
ryanmen, has already been ruled out,
so it is at least a little better.
For reference, here is the suji chart in table form:
| Discarded Tile | Suji |
|---|---|
4 |
1 and 7 |
5 |
2 and 8 |
6 |
3 and 9 |
1 and 7 |
4 |
2 and 8 |
5 |
3 and 9 |
6 |
Safer Suji
Dora 
Let us use this riichi as the example.
Example 1
Tsumo
Since has already been discarded, both and are suji.
So which one should you discard first?
- can hit: a tanki, a shanpon involving , and also
or 
- can hit: a tanki, or a shanpon involving
Since no longer carries extra penchan or kanchan risk, it is clearly safer.
So here, the correct discard is .
Some people might wonder whether a tanki really happens.
In composite shapes like this, it is entirely possible.
Theory
Terminal tiles are safer than other number tiles, because they have no kanchan or penchan risk.
Dora
Figure 2
Tsumo
Here, you are holding both and , and both count as suji.
- can hit: a tanki, a shanpon involving , and
- can hit: a tanki, a shanpon involving , and
However, is already fully visible in all four copies, including the dora indicator,
so a kanchan wait cannot exist.
On the other hand, because the dora is , a kanchan wait looks very plausible.
If someone had a double-kanchan shape like ,
they would often cut and riichi.
Shapes like the following are also entirely possible:
So is not just a "suji, so it is safe" tile at all.
It is actually a very dangerous tile.
From Figure 2, the correct discard is .
Theory
Tiles next to the dora are dangerous even when they are suji. Suji-trap riichi is also very common.
Original Japanese page: http://beginners.biz/mamori/mamori04.html