XY-Chain
A chain of pairs that starts and ends on one digit forces an end.
Learn the XY-Chain Sudoku technique: a chain of bivalue cells that begins and ends on the same digit forces one end to be it, so cells seeing both ends lose it.
How to use it
- Link bivalue cells. Find a run of cells that each hold exactly two candidates, where each cell shares one candidate with the next and the two ends both contain a target digit z.
- Follow the forced alternation. Assume the first cell is not z: it must then be its other candidate, which forces the next cell, and so on until the last cell is forced to be z.
- Eliminate z from cells seeing both ends. So the first end is z or the last end is z — either way any cell that sees both ends cannot be z, and z is removed there.