Hidden single

Easy

A hidden single is a digit that has only one place left in a row, column or box. It’s the first technique everyone learns, and on Easy puzzles it’s the only one you need: no notes required.

How it works

Pick a box and a digit it still needs. Every copy of that digit already on the board rules out the empty cells in its own row and column. If that leaves only one empty cell in the box, the digit goes there.

It works the same way in a row or a column: look at where a digit could still go along the line, and cross out the cells it can’t.

The easiest case: a full house

When a row, column or box has just one empty cell, the missing digit goes there. Players call it a full house, and late in a puzzle there are plenty.

Why it’s called hidden

Looked at on its own, the cell might still allow several digits. It’s only from the digit’s point of view, scanning the box for places it could go, that the answer shows. A cell with only one digit left is a different technique: the naked single.

How to spot them

• Take one digit at a time and sweep it across the whole board. Start with the digit placed most often: it rules out the most cells.
• Look at boxes first. Lines from the boxes beside them cut through a box from two directions, which is where most hidden singles hide.
• Each digit you place rules out cells in its row, column and box, so look there again first: one find often leads straight to the next.