Killer pairs
Two cells in one cage that share the same two candidates lock those digits — like a naked pair, but driven by the cage sum.
What it is
When a cage's combinations force two cells to hold exactly the same two digits, those digits are tied to that pair. Since both cells lie in the same cage — and often the same row, column or box — the two digits can be removed from every other cell in the shared unit.
How to spot it
Find cages where the remaining combinations leave two cells with an identical pair of candidates, then check whether both cells share a row, column or box.
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
- 6/1, 8/1
- Digits
- 7, 8
- Follows from it
- 2/1, 5/1, 9/1
Press a marked square to point it out on the board above.
Why it matters
The two digits must occupy those two cells in some order; no third cell can take either of them. This is the naked pair argument, with the cage sum rather than pencil marks doing the restricting.
Practise now
The practice puzzle is picked so that this technique is what gets the solve moving again.
Frequently asked questions
How is this different from an ordinary naked pair?
The mechanism is the same; the source differs. Here the cage sum forces the pair, so it can be spotted before the pencil marks reveal it.
Do both cells have to be in the same box?
They must share a unit for the elimination to apply — row, column or box. Cage membership alone allows elimination only within the cage.
Are there Killer triples too?
Yes, on the same principle with three cells and three digits. They are rarer and usually appear only in hard puzzles.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
Play ExpertStuck 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.