Naked Single

The simplest technique of all: a cell where only one number is left.

What is a naked single?

A naked single is an empty cell where only one number is still possible. Every other number from 1 to 9 already appears in the cell's row, column or box.

It is called naked because the answer is out in the open: look at the cell, cross off what it can't be, and one number is left.

the cell its row, column and box

Example from a real puzuru puzzle. Rows and columns are numbered 1 to 9 from the top left.

The example

Look at the cell in row 1, column 7 (marked ?). Its row, column and box are shaded.

Between them, those three units already contain every number except 1. So this cell has to be 1.

Notice that 1 could still go in other cells of the row, the column and the box, so you would not find this one by sweeping for 1. You find it by looking at the cell.

How to find naked singles fast

Look for cells whose row, column and box are crowded. The more numbers around a cell, the fewer it can hold.

Count what's missing: list the numbers already in the row, add the column, add the box. If eight different numbers are there, the ninth is your answer.

With notes on, a naked single is simply a cell with one candidate left.

Naked single vs hidden single

A naked single starts from a cell: which numbers can go here? A hidden single starts from a number: where can this number go in this row, column or box?

You need both. Together they solve every Easy and Medium puzzle on puzuru.

Practice on Medium puzzles →

← All strategies　Next: Hidden Single →

FAQ

What is the difference between a naked single and a hidden single?

A naked single is a cell with only one possible number. A hidden single is a number with only one possible cell in a row, column or box.

Is a naked single the same as a sole candidate?

Yes. Some books call it a sole candidate or a single candidate.

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