Forcing Chain
Assume a candidate, follow the consequences, and see whether both assumptions lead to the same result.
What it is
A forcing chain tests a cell's candidates by following each to its logical conclusion. If every assumption forces the same digit into some other cell, that digit is confirmed regardless of which was right. If one assumption leads to a contradiction, that candidate can be eliminated. Unlike guessing, nothing is written down until the conclusion is certain — the chain proves it.
How to spot it
Choose a bi-value cell in a dense area, since short chains are easier to verify. Assume the first candidate and follow the forced placements as far as they go, then repeat with the second. Compare the outcomes: a shared conclusion or a contradiction is your result.
Why it matters
Forcing chains solve puzzles that no pattern-based technique can crack. They are the most general method short of trial and error, and they are what solver programs fall back on. The cost is effort: chains are slow to trace and easy to get wrong.
Frequently asked questions
Is a forcing chain just guessing?
No. Guessing means writing a digit and hoping. A forcing chain follows both possibilities and only concludes when they agree or one fails — the result is proven either way.
Why is there no fixed example for this?
Because a forcing chain is a method rather than a pattern. Its shape depends entirely on the position, so a single grid could not represent the technique fairly.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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Type the position into the solver and it walks the whole solution, naming the technique behind every step — including this one.
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