A chain of squares with two options each, where each square forces the next. If both ends of the chain offer the same number, one end takes it, and a square that sees both ends can't.
123456789
123456789
19
6
7
5
4
8
8
7
4
5
9
15
5
35
4
8
9
2
1
3
1
9
5
4
3
6
1
59
1
2
7
8
6
6
1
4
2
7
4
35
9
2
8
6
1
The chainLoses a pencil mark
Reading the chain
Each link is a square with exactly two options, and each step uses the number the last square didn't take. Chains can run for many squares; this one has five.
How to spot one
Start from a square with two options, assume it isn't one of them, and follow the squares that forces. If you reach a square that offers the same number, you have a chain.
What it gives you
Removals where the ends' lines cross. XY-Chains appear on extreme boards, once the shorter wings have run out.
Practise XY-Chain
Boards that need it, with the owl's hints when you're stuck.