Practise: Killer Naked Single
This trainer begins exactly where ordinary Sudoku leaves a real choice and the cage removes it. Check both lists yourself before revealing the proof.
The blue cell shares a small 5-cage. First list its normal Sudoku candidates, then keep only digits that can still fit the cage. One digit remains.
Your turn first
Spot the idea before the hint explains it
- 1. Look
- 2. Check
- 3. Proof
Look at the marked starting cell, cage outline and sum. Which digit or combination is forced, and what visible sum fact supports your idea?
How to spot it
Look for a short, restrictive cage near a cell with several pencil marks. A two-cell cage of 5, 6 or 17 is especially useful: its few legal pairs often leave only one digit that also respects the row, column and box.
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 solver