Lesson 5 of 8
Hidden pair
A hidden pair is two digits that, within one unit, can go in the same two cells and nowhere else. Those two cells have to hold those two digits, whatever else is written in them, so every other candidate in those cells can be removed.
- Level
- Intermediate
The pattern
What it does.
It's the mirror image of a naked pair. A naked pair takes two digits out of the rest of the unit; a hidden pair takes the rest of the candidates out of two cells.
Both are about the same two digits and the same two cells. Which one you find first depends on whether you're reading the cells or the digits.
Example
A worked example.
Choose a unit and two digits
Work on one row, column or box. Pick a pair of digits that the unit hasn't placed yet.
Find where each digit can go
For each of the two digits on its own, list the unsolved cells of the unit that can still take it. Judge this from the grid rather than from your pencil marks.
Look for two cells in common
If both lists are the same two cells, the digits are hidden in them. If either list holds a third cell, there's no hidden pair here.
Clear the two cells
Remove every other candidate from those two cells, leaving the pair behind. Nothing is placed yet, but the two cells are now a naked pair for the rest of the unit.
Spotting it
How to spot it.
- A unit with crowded cells and two digits that appear in very few of them. The mess is the point: the pair hides behind the candidates that don't belong.
- Work through the digits as well as the cells. Hidden pairs are invisible from a single cell, which is why the naked pair is usually found first.
- Boxes crossed by two nearly finished lines, where two digits have been squeezed into the same corner.
Where it stops working
A hidden pair needs a complete candidate list to see at all, and it removes candidates rather than placing digits. It's also the mirror of a naked pair, so if a unit already shows a naked pair you won't find a hidden one among the same cells: it's the same discovery, read from the other side.
Keep going
Related topics.
New to sudoku? The rules take two minutes to read, and the first-puzzle walkthrough covers the basics that come before any of these.
Practice it in Sudoku Mori.
Hidden pair is lesson 5 in the game, with a worked example and three practice puzzles. Lesson puzzles are separate from your daily ones, and you can redo any lesson whenever you like.