Proven Sudoku
Unit Forcing Chain
Also called House Forcing Chain, Unit Forcing Chains.
New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.
What it means
Take one number in one row, column or box and try each home it could have. Follow each one. Whatever holds in every case is true.
The exact rule, for stronger players
The rule, stated exactly: Take every possible position of one digit within one house as a starting assumption. If all force the same conclusion, it holds.
What it removes: Apply the common conclusion.
Why it works: The digit must go somewhere in the house, so the branches are exhaustive.
Worked examples
None yet. This pattern does not turn up in our daily boards, so there is no real example to show. We would rather wait than draw a fake one.
Learn these first
- Forcing Chain (general form)Pick something and assume it.
Related chains
- X-ChainA chain about one number only.
- XY-ChainA chain of squares that each hold exactly two numbers.
- AIC with AHS NodesThe same as above but using sets of numbers that are hiding rather than sets of squares.
- AIC with ALS NodesA chain that can step through a small set of squares holding one more number than it has room for.
- Alternating Inference ChainA chain that switches between two kinds of step.
How often does it come up?
Our rough estimate: as for cell forcing chains.