Expert

AIC (alternating inference chain)

What it is

A chain that alternates strong and weak links between candidates, also across different digits.

How to spot it

Combine links of one digit in a unit with links between two digits in one cell. With a strong link at each end, one of the ends is true.

Example

Basis
Removed / placed

If r1c1 isn't 2, then r1c5 is 2, r5c5 isn't 2 so it's 7, r5c1 isn't 7 so it's 2. Either r1c1 or r5c1 is 2, so 2 is removed from the rest of column 1 – here r3c1 and r8c1.

Used from level: Expert