No repeats in a cage
A digit appears at most once per cage — even when the cage stretches across two boxes where the digit could otherwise repeat.
What it is
Beyond the usual row, column and box constraints, Killer Sudoku adds one more: within a cage every digit is distinct. This matters most for cages that span two boxes, where standard Sudoku would happily allow the same digit twice.
How to spot it
Find cages that reach across a box or row boundary, then check whether a candidate digit already appears elsewhere in the same cage. If it does, it can be struck.
Worked example
Step through the example. First the position, then the cage the argument rests on, then the candidates, and finally what follows from it.
Step 1 of 4: The starting position
- Pattern cells
- 6/6, 6/7
- Digits
- 3, 4
- Follows from it
- 6/6, 6/7
Press a marked square to point it out on the board above.
Why it matters
The rule is part of the Killer definition and is what makes cage sums meaningful: without it a cage of 10 over two cells could be 5+5, and almost no sum would restrict anything. With it, every cage sum implies a set of distinct digits.
Practise now
The practice puzzle is picked so that this technique is what gets the solve moving again.
Frequently asked questions
Do all Killer variants use this rule?
Nearly all published Killer Sudoku do, and it is the standard here. A handful of older variants allow repeats; those state it explicitly.
Where does the rule bite hardest?
In cages that span two boxes. Inside a single box the box rule already forbids repeats, so the cage rule adds nothing new there.
How does it interact with cage sums?
It is what makes them restrictive. Because digits must differ, a two-cell cage of 10 can be 1+9, 2+8, 3+7 or 4+6 — but never 5+5.
Put it to use
Where you will meet it
Medium grids are where this move starts to matter: the obvious cells run out and the pattern takes over.
Play MediumStuck on a grid of your own?
Type the position into the solver and it walks the whole solution, naming the technique behind every step — including this one.
Open the solverFinished reading? Mark it done and the learning path remembers where you got to.