What the dead wall is
After the wall is broken, the tiles on one side of the break are reserved rather than used for normal draws. In Japanese play exactly seven stacks (fourteen tiles) form the dead wall, and the live wall is shortened as replacement tiles are taken. Chinese styles reserve a smaller number of tiles or draw replacements from the far end of the wall.
The live wall ends when only the dead wall remains. That is why hands sometimes end in a draw even though tiles are still visibly on the table.
Replacement tiles
When a player declares a kong, they hold an extra tile and need one more to complete a fourteen-tile hand. They draw a replacement from the dead wall. In Chinese styles the same happens when a flower or season is drawn.
Winning on a replacement tile is a special event with its own scoring element in many styles — "win on a kong replacement" in Hong Kong play, rinshan kaihou in Japan.
Dora indicators
In Japanese mahjong the third tile from the end of the dead wall is turned face up as the dora indicator. The tile one step after it in sequence is the dora, a bonus tile worth one han each in a winning hand. Each kong reveals another indicator, increasing the potential value of every hand.
Players who win after declaring riichi also get to look at the tile beneath each indicator — the ura-dora — which may add more value.
Why it matters to strategy
The dead wall affects how many draws remain, which in turn affects how long you can wait for a difficult hand. Japanese players keep a close eye on the number of tiles left in the live wall, because a drawn hand pays ready players at the expense of unready ones.
It also makes kongs double-edged: each one gives you a fresh tile and, in Japan, adds dora that could help any player.
