Sotogawa

Sotogawa tiles (lit. outside tiles) are tiles further away from 5 than a discarded tile.

When a number tile is discarded early, same suit tiles ±2 towards the outside become safer tiles due to sotogawa. For example, if 3-pin is discarded early, then 1-pin and 2-pin become safer. This theory only applies to early discarded tiles, and only applies to tiles within ±2 of the discard. Tiles that are ±3 away are suji, while tiles ±4 away are too distant (and more dangerous due to ura suji).

Theory

Assuming you are playing against experienced players, sotogawa is a bit less safe than suji, but still on a comparable level to suji. A tile that is affected by both sotogawa and suji (e.g. a 2 after both 3 and 5 are discarded) is safer than a tile that is one or the other.

The concept of matagi suji is the main reason why sotogawa is effective. Matagi suji states that an opponent is less likely to cut a 3 from "233" or "344". So, if they cut a 3, it is less likely for them to have "23" (waiting on 1-4) or "34" (waiting on 2-5) remaining; this makes 1-4 and 2-5 safer. Of those, 1 and 2 are safe "sotogawa" tiles, while 4 and 5 are not safe as an opponent could still have a "56" or "67" ryanmen. Thus if 3 is discarded, 1 and 2 are safer.

Generally, "early discards" are the first six discards of the game, excluding a riichi declaration tile. If a player declares riichi early, then the sotogawa of the riichi declaration tile become more dangerous instead of less dangerous. The earlier the discard, the more safe sotogawa is.

Sotogawa is less effective against new players, since this theory relies on players having strong tile efficiency.