Forcing Chain Variants (Digit, Cell, Unit, Quad)
Named by what's exhaustively tested — every candidate of one digit, every candidate of one cell, every cell of one unit, or every cell across four related units — each variant follows every branch to see if they all agree on some outcome.
The solver does not attempt this technique, so Smart Hint will never suggest it. It is documented because you may still want to spot it yourself.
How it works
- 1A Cell Forcing Chain tests every candidate of one cell in turn — here A1's 2 and its 6 — and follows each one's consequences on its own.
- 2Start with A1 = 2: D1 shares row 1, so its 2 goes and it becomes 9, and the cascade runs on from there. Then start over with A1 = 6 and follow that instead.
- 3If both runs end up agreeing that E5 can't be 8, the elimination holds whichever is true. Digit, Unit and Quad Forcing Chains do the identical thing, starting from one digit's every occurrence, one unit's every cell, or four related units at once.
- First colour in the chain
- Where the conclusion lands
- Candidate ruled out