Ura suji
Ura suji is an older theory which suggests that: when a middle tile is discarded relatively early, an adjacent tile and its "suji" become more dangerous. While named similarly to the defensive technique of suji, they are mostly unrelated.
In the modern day, ura suji is often disregarded. While it has some truth - the tiles ura suji indicates do become somewhat more dangerous[1] - it rarely impacts decision-making. If folding, you wouldn't discard unsafe tiles (whether normal tiles or ura suji), and if pushing, it's often worth doing so despite ura suji's added danger.
Theory
Say an opponent had a "35" kanchan in their hand. If a 2 is drawn, then 5 would be discarded from the "235", leaving just a "23" ryanmen shape. The "23" ryanmen waits on 1-4, thus making 1-4 of the same suit more dangerous. Therefore, a discarded 5 makes 1 and 4 into ura suji.
Similarly, with a 24 kanchan, if a 5 is drawn, then a 2 is discarded, leaving "45" ryanmen (waiting on 3-6, making 3 and 6 into ura suji). However, since lone 2's are often discarded early, anyways, it is significantly less effective.
Ura suji becomes more effective when the discard is earlier and when the discarded tile was a middle tile. Also, ura suji's danger increase is relatively higher for 1/9 and 2/8 tiles.
If a tile would be both ura suji and suji/kabe, suji/kabe takes priority. Ura suji makes it more likely for a ryanmen to appear, but both suji and kabe prevent ryanmen waits entirely.
Table
Except for discarded 5, a discarded tile makes the 1-away tile (towards the inside) and the 4-away tile ura suji. In other words, "the adjacent tile (towards the inside) and its suji" become ura suji. For example, a discarded 3 makes 4 and 7 into ura suji.
For discarded 5, both possible adjacent and 4-away tiles are more dangerous.
Discard | Ura suji tiles |
---|---|
& | |
(Early discarded 1/2/8/9 tiles are not particularly noteworthy, since these tiles are often discarded early anyways.)
Why ura suji is not very useful
Not as dangerous as previously thought
Ura suji originates from the pre-internet era, where statistics were harder to obtain. Modern knowledge suggests that the case where ura suji is dangerous (have "35" draw 2 -> discard 5, have "23") was less common than previously thought. This is because:
- Ura suji assumes that the "35" kanchan existed, when it could have been from a number of different shapes ("57" kanchan, complete "556" shape...)
- Ura suji assumes the opponent is going for maximum tile efficiency. The opponent could be holding to a safe tile or going for value.
- Even if #1 and #2 are both true, there is still a significant chance that the "23" would have completed before tenpai.
Also note that the 1-away tile is affected by matagi suji. For example, an early discarded 4 makes 5 into both ura suji (common to discard 4 from 467, making "67" ryanmen more common) and matagi suji (not common to discard 4 from 344, making "34" ryanmen rarer). Overall, the 5 still ends up being more dangerous than normal, but the effect of ura suji is reduced.
Not dangerous enough to change our actions
Due to the above reasoning, ura suji is often disregarded. It is still true that, if an opponent discards a 5-pin early, then a non-suji 4-pin is at least somewhat more dangerous than normal. However, a non-suji 4-man, 5-man, 4-sou, etc. would also be quite dangerous, even if not as dangerous.
- When folding, you should look for the safest tiles first. Knowing whether an already dangerous tile is more dangerous is not helpful to you. Therefore, you wouldn't discard a non-suji tile unless you had to, even then you wouldn't discard a middle tile. Thus, you'd avoid discarding a 4-pin whether it was ura suji or not.
- When attacking, you would usually discard ura suji anyways, because an ura suji is not so much more dangerous than a regular non-suji tile.
Because knowing about ura suji would not change your behavior when folding, and rarely changes your behavior when attacking, it is rarely useful to know about ura suji.
That being said, there are a few cases where ura suji helps. If an opponent declares riichi early, and you have to decide between a regular 1 or ura suji 1 for defense, the regular 1 will be safer. Even then, you wouldn't discard the regular 1 unless you had nothing safer.
Aida yon ken
Aida yon ken is a similar theory to ura suji. It states that: when an opponent has discarded two tiles that are 5-away (forming a 4-wide gap), the remaining non-suji tiles inside are more dangerous.
Discards | Aida yon ken |
---|---|
The added danger comes from two factors:
- Suji counting:[2] discarding both 1 and 6 eliminates many ryanmen intervals (1-4, 3-6, 6-9 are suji; 4-7 and 5-8 are potentially affected by sotogawa. The only ryanmen interval left in the suit is 2-5.). This makes the remaining ryanmen more dangerous.
- Ura suji: a discarded 1 makes 2-5 ura suji, and a discarded 6 also makes 2-5 ura suji. Therefore, after discarding 1 and 6, 2-5 is "double ura suji", or aida yon ken.
Like ura suji, it's true that the aida yon ken tiles are more dangerous than normal, but it can often be ignored in actual play.
- Much of the danger can be explained by suji counting, which applies to any non-suji tile, not just aida yon ken. Therefore, you don't need to know about aida yon ken to know that the tiles are dangerous.
- When folding, you look to find the safest tiles, not which of your dangerous tiles are more dangerous than normal. When pushing, the extra danger is often not enough to change the decisions.
References
- ↑ https://pathofhouou.blogspot.com/2020/08/analysis-ura-suji-and-matagi-suji.html
Note that the table only lists the % chance for that ryanmen wait. For instance, if 5 was discarded and riichi is called after, there is an 8.4% chance for a 1-4 ryanmen to exist (172% as much as normal). This doesn't mean that 1-4 is 172% as dangerous, though, as there could be other waits. - ↑ https://pathofhouou.blogspot.com/2020/07/guide-defense-techniques-aida-yon-ken.html