Lesson 05 of 08 — Intermediate
Hidden pair technique
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.
What it does
It is 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 are reading the cells or the digits.
A worked example
Choose a unit and two digits
Work on one row, column or box. Pick a pair of digits that the unit has not 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 is 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.
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 do not 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.
A hidden pair needs a complete candidate list to see at all, and it removes candidates rather than placing digits. It is also the mirror of a naked pair, so if a unit already shows a naked pair you will not find a hidden one among the same cells: it is the same discovery, read from the other side.
Practise it in Sudoku Mori
The game teaches every technique in this order, each with a worked example and three puzzles of its own. Lesson puzzles are kept apart from the daily ones, and any lesson can be revisited.
Keep reading
All eight: Single candidate · Hidden single · Locked candidates · Naked pair · Hidden pair · Naked triple · Hidden triple · X-Wing