X-Chain
A single-digit chain of alternating links forces one end to be true.
Learn the X-Chain Sudoku technique: a single-digit chain alternating strong and weak links forces one end to hold the digit, so cells seeing both ends lose it.
How to use it
- Chain strong and weak links. Pick a digit and build a chain that alternates strong links (a unit where the digit has only two spots) with weak links (two spots sharing a unit), starting and ending on a strong link.
- Read the endpoints. Because the chain starts and ends with a strong link, at least one of its two endpoints must hold the digit — you do not know which, but you know one of them does.
- Eliminate from cells seeing both ends. Any cell that sees both endpoints sees the digit no matter which end is true, so the digit can be removed from it.