- Look at the highlighted cells with 5.
- Box 1: 5 fits in only one cell.
- So this cell must be 5.
Hidden Single in sudoku
A Hidden Single is a number that has only one cell left in a row, column or box. The cell itself may still allow other numbers, which is why the single is "hidden": you find it by following the number, not the cell.
Or scan with your phone
How to spot it
Take one number and one box. Cross out, in your head, every cell of the box that shares a row or column with that number elsewhere. If one empty cell is left, the number goes there. Do the same along rows and columns. Easy to Expert puzzles in the game need nothing beyond singles, and most of the singles are hidden ones.
Why it works
Each row, column and box must hold every number once. If a number can go in only one cell of a box, no other cell can take it, so that cell must.
On a real puzzle
This is Hidden Single on its practice puzzle, at the first step that needs it. The game's hint explains the step like this:
Practice it
This Easy puzzle is solved with singles alone, from the first number to the last. If you get stuck, the hint shows the next one and names it.