When you put a stone in atari and it runs, you need a way to keep it caught. There are two, and choosing wrongly can lose a game outright.
The ladder
Atari from one side, the stone extends, atari from the other side, it extends again. The chase runs diagonally across the board in a staircase, with the running stones always at one liberty. If the pattern reaches the edge, everything is captured.
The catch is in the word 'if'. A ladder must be read all the way to the edge before you start it, because a single enemy stone anywhere along the diagonal path changes everything.
Ladder breakers
If any friendly stone of the running player sits on or near the ladder's path, the chase reaches it, the running group connects, and suddenly it has liberties. Worse, all the atari stones you played are now a weak, broken line and the position collapses.
This is the single most punishing mistake in go tactics, because the damage is spread across the whole board rather than confined to one corner. Playing a ladder that does not work is far worse than not playing it at all.
Read the ladder to the edge, every time
Trace the diagonal path with your eye all the way to the board edge and check for any enemy stone within one point of it. If there is one, the ladder fails.
Ladder breakers as a move
Because ladders depend on distant stones, a move played far away can make a ladder work or fail. Playing such a move is called a ladder breaker, and it functions as a threat: if the opponent ignores it, you capture with the ladder; if they answer it, you have gained a free move elsewhere.
The net
A net — geta — catches a stone by surrounding the area it would escape into, rather than chasing it directly. The stone is not in atari, but every escape route is covered. It costs one more stone than a ladder and it does not depend on anything happening elsewhere on the board.
That independence is the whole point. If you are unsure whether a ladder works, play the net. A net that works is worth more than a ladder that might not.
Choosing between them
| Ladder | Net | |
|---|---|---|
| Stones used | Fewer | One more |
| Depends on distant stones | Yes | No |
| Result if it fails | Disastrous | Usually just a small loss |
| Outside influence gained | Large | Moderate |
Strong players use ladders when they have read them and want the extra influence from the chasing stones. Everyone else should default to the net.
Practising
Ladders are the easiest thing in go to practise against a computer, because the pattern is completely mechanical. Set up a chase on the 9×9 board and follow it to the edge a few times until the diagonal path is obvious at a glance. Then start checking for breakers before you commit.