Technique

Forcing Chains

When a cell's few candidates each get followed forward through their own chain of forced consequences, and every branch agrees on some outcome elsewhere, that outcome must be true regardless of which candidate is actually correct.

💡 What It Is

A 'try both, see where they agree' technique: instead of finding one direct pattern, it traces the knock-on effects of each candidate in a cell separately.

🔍 How To Spot It

Pick a cell with few candidates. For each candidate, follow what it forces elsewhere on the board (singles that become forced as a result, and so on). If every branch lands on the same conclusion - a cell taking the same value, or a candidate disappearing in both - that conclusion holds no matter which branch is real.

✅ Why It Works

Exactly one of the starting candidates has to be correct. If both possible worlds agree on something, that something is true in whichever world actually happens.

📝 Worked Example

R3C3 could be 2 or 9. Assuming 2 eventually forces R7C7 to be 5 through a chain of singles; assuming 9 also eventually forces R7C7 to be 5 through a different chain. Either way R7C7 is 5, so it can be placed now without knowing which branch is true.

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