Next I'll introduce two very important concepts in Japanese mahjong: suji-pai and kabe.
In Japanese mahjong, 60% of tenpai in riichi are ryanmen waits. Therefore, if it is certain that some tiles cannot be ron by ryanmen tenpai, they are considered to be safe.
Suji-pai(Degree of safety: S~B)
We'll use the previous riichi image as an example. If the player who declared riichi discards 5 pin, then if you discard 8 pin, then the opponent can't ron on your discard even if he holds 67 pin and is waiting on 58 pin.(According to the furiten rule.) To ron on 8 pin, the only possibility is shanpon and kanchan, therefore the probability of dealing in is lower. Similarly, 2 pin, 9 pin, 9 sou are safer due to the fact that they're suji-pai.
If 4 is discarded, 1 and 7 are the suji-pai.
If 5 is discarded, 2 and 8 are the suji-pai.
If 6 is discarded, 3 and 9 are the suji-pai.
We normally remember these three groups as 1-4-7, 2-5-8, 3-6-9.
It is worth noting that, even if 1 sou is discarded, 4 sou is not a suji-pai, as he could still be waiting on 4-7 sou.
By the same token,
If 1 and 7 is discarded, 4 is a suji-pai.
If 2 and 8 is discarded, 5 is a suji-pai.
If 3 and 9 is discarded, 6 is a suji-pai.
The 456 here is called double suji-pai. If only one of the suji is discarded, it is called half suji-pai.
Difference between degree of safety for different suji-pai
There are many suji-pai introduced above, but their degree of safety are all different.
For example, the suji-pai of 9 pin, as the chances of ryanmen is eliminated, then it can only be tanki or shanpon tenpai, the chances of safety is similar to word tiles. If you see two of the 9 pin already discarded, 9 pin can only be hell wait, then the degree of safety is similar to that of tanki word tiles. If you can see three of the tiles, then it's 100% safe.
Hence, suji-pai of 1 and 9 can be said to be the safest among all suji-pai.
For suji-pai of 2 and 8, not only can it be shanpon and tanki, it can also be kanchan for 13 and 79. Therefore the degree of safety is lower.
For suji-pai of 3 and 7, there can be penchan of 12/89 and kanchan of 24/68. Therefore, the risk is much higher. The kanchan could even be part of a tanyao hand, increasing the degree of harm to you.
As for the double suji-pai of 456, as there can only be kanchan, the degree of safety is similar to suji-pai of 28.
In summary, the degree of safety for suji-pai in order is:
Suji-pai of 19 >> Suji-pai of 28 == Double suji-pai of 456 > Suji-pai of 37
The next round will be about another important concept: Kabe.
(To be continued)