Inference Chain
Expert Removes candidates Used in: Sudoku, Killer Sudoku
An Inference Chain weaves strong and weak links across different digits. Whatever the truth values along it, one of the two endpoints must hold — letting you eliminate any candidate that sees both ends.
How to spot it
An alternating chain of strong and weak links, across one or several digits.
What you can conclude
Eliminate any candidate that sees both endpoints of the chain.
Worked example
Alternate strong and weak links to an endpoint pair; whatever the chain does, one end must hold, so cells seeing both ends are pruned.