Kabe
Kabe 「壁」 is a defensive tactic that takes into account the number of tiles available. If you can see multiple copies of a tile, then an opponent is less likely to have that specific tile. If you can see all four copies of that tile, an opponent can never have that tile. This fact makes other nearby tiles safer to discard.
Kabe can also help with tile efficiency. As tiles are played, it becomes less likely for you to draw said tiles.
Visible Tiles
Visible tiles include:
- Tiles in the discards
- Tiles in the dora indicator(s)
- Tiles in your own hand
- Tiles used for your opponent's tile calls
Only four copies of a tile exist. So if most or all of the copies of a tile are visible, then you can gain information about the other players' hands.
A call for kan automatically reveals 4 tiles, so kan calls are particularly subject to kabe.
Defense
Kabe does not make a tile 100% safe, but it can help determine which tiles are less likely to deal in.
No chance
If all four copies of a tile are visible, then no copies of that tile can be hidden inside an opponent's hand. Thus, certain hand compositions become impossible.
For example, if all four 
are visible, then it is impossible for an opponent to have 3-pin. Therefore, it is impossible for an opponent to have a wait pattern of:
| Pattern |  |
 |
|
|---|---|---|---|
| Waiting for | and |
and  |
|
Therefore, and are much safer than normal - these are considered "no chance" tiles. It is impossible for an opponent to complete a sequence with these tiles. It is still possible for an opponent to win off a shanpon (triplet wait) or tanki (pair wait), though these types of wait are considerably rarer.
Regarding and , these tiles are only a bit safer than normal. It is still possible for an opponent to have a sequence wait on these tiles, such as ( 
) and ( 
) respectively. These tiles are not considered "no chance".
In general: if all four copies of a numbered tile are visible, then any nearby tiles closer to the end become safer. So a kabe of 4-pin would make 2-pin and 3-pin no chance tiles; a kabe of 2-pin would make 1-pin a no chance tile. Also, having two kinds of kabe will make certain tiles in-between safer: a kabe of 3-pin and 7-pin will make 5-pin safer (but not 4-pin or 6-pin).
One chance
One chance occurs when three copies of a tile are visible. Otherwise, the concept is similar to no chance. That is, when three copies are visible, then it is less likely for an opponent to have the fourth copy hidden in their hand. For example, if you can see three copies of 3-pin, then 1p and 2p are less likely to be an opponent's winning tile.
Of course, a one chance tile is less safe than no chance, but still safer than any random tile. However, one chance becomes more dangerous as the game progresses (since it becomes more likely that the 4th copy of the tile is in the opponent's hand).
Usage
A no chance tile is usually safer than suji. A no chance tile is immune to ryanmen and kanchan waits, while a suji tile is only immune against ryanmen waits. In addition, kabe applies to all players, while suji has to be checked for each individual player. Of course, it is possible to apply both defensive techniques when defending against an opponent. One chance tiles are less safe than suji.
Like suji, kabe implies that an opponent has a ryanmen (or kanchan) wait. So if an opponent is going for a hand like toitoi or chiitoitsu, kabe doesn't apply.
Tile Efficiency
By counting the number of visible copies of a tile, you can deduce how likely a certain tile group is to form. For example, when two of a type type are visible, then no other player can form a triplet. This particularly applies with honor tiles. The same logic may be applied to sequences; e.g. as 3-man tiles become visible, completing a 12-man penchan becomes less likely.
Thus, it may hinder a player's hand from reaching tenpai, completing, or having certain yaku. This is particularly the case for yakuman. In the case of kokushi musou, if all four of a tile type required for that yakuman, then the possibility for any player completing the hand is completely nullified.