Expert

X-Chain

What it is

A chain of one digit that alternates strong and weak links – one of its ends is the digit.

How to spot it

Follow one digit: a strong link (only two places in a unit), then a weak link (same unit), then a strong link again. Cells that see both ends can't hold the digit.

Example

Basis
Removed / placed

4 has only two places in column 1 (r1c1, r7c1) and in column 8 (r7c8, r2c8), and r7c1 and r7c8 share row 7. Either way, r1c1 or r2c8 is 4, so 4 is removed from r1c9 and r2c2, which see both ends.

Used from level: Expert