Grafite Sudoku
The hidden pair in sudoku
A hidden pair is two numbers that, in a row, column or box, can only sit in the same two cells. Since those two numbers must go there, the two cells cannot hold anything else.
It is "hidden" because the two cells have other pencil marks covering it: you see the pair by looking at the numbers, not at the cells.
In the example, in this row 3 and 8 only appear in the pencil marks of columns 2 and 7. So those two cells are 3 and 8 (in an order still to find), and the other marks, 1, 5 and 6, can be erased.
How to find it
- Write pencil marks in every cell of a row, column or box.
- For each number, count how many cells it appears in.
- Look for two numbers that appear in exactly two cells, always the same two.
- Erase the other marks from those two cells.
After the clean-up a single often shows up in a nearby cell: that is the reward.
In the app: "Hidden pair" lesson in the Sudoku Academy
In the Sudoku Academy of Grafite Sudoku this technique is an interactive lesson on the grid, with exercises from real puzzles.