Hidden Pair

Level: first needed in Master puzzles. In one line: Two numbers that can only go in the same two cells of a unit; every other candidate in those cells is removed.

A Hidden Pair is two numbers that, within a row, column or box, can only go in the same two cells. Those two cells must hold those two numbers, so every other candidate in those two cells can be removed.

It is "hidden" because the two cells usually have other candidates too, which disguises the pair. After the clean-up it turns into a naked pair, and often a single appears nearby.

Example from a real puzzle

This position comes from an Extreme puzzle in Sudoku Arena, at the exact moment the Hidden Pair is the next logical move. Cells are named by row and column: R3C4 means row 3, column 4.

{1,7} can only go in R4C9, R7C9 within column 9. Result: remove 5 from R4C9, 8 from R7C9.

Play this puzzle and look for the move yourself. It's a practice game, so it doesn't count towards your stats.

How to spot it

1. Pick a unit and, for each missing number, note which cells could take it.
2. Look for two numbers that each have exactly two possible cells, and the same two.
3. Erase every other candidate from those two cells.

Tips

• Work number by number rather than cell by cell: hidden subsets are about where numbers can go.
• Hidden pairs are the signature technique of Master puzzles in Sudoku Arena.

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