Hidden single
A digit fits in only one cell of a row, column or box.
A technique for Easy puzzles. Play a Easy puzzle
What it does
Pick a digit and a row, column or box. If there is only one cell in it where the digit can still go, the digit goes there, even if that cell could hold other digits too. It is called hidden because the cell itself may still look open.
How to spot it
Scan one digit at a time. For a box that doesn’t have it yet, cross out every cell that sees that digit in its row or column. When only one cell is left, you have found it. Then do the same for rows and columns. A unit with a single empty cell left, a “full house”, is the easiest case.
What it lets you do
You place the digit. No other cell of that unit can take it, so this is safe even when the cell has other candidates.
Try it
Find the Hidden single on this board. Pick its cell, then choose its digit.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Easy level; any Easy puzzle may need it too.