Sudoku technique guide

Almost Locked Set Sudoku Technique

An Almost Locked Set is a small group of cells with one more candidate than cells. This guide explains the idea in plain English, when to look for it, and the common mistake to avoid.

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At a glance

Difficulty

Advanced

This is where Almost Locked Set normally sits in a human solving path.

Family

ALS techniques

It belongs to the als techniques group of Sudoku logic.

Main action

Usually remove candidates

Most technique moves reduce notes first. That often reveals a simpler placement afterwards.

What is Almost Locked Set?

An Almost Locked Set is a small group of cells with one more candidate than cells.

The important thing is that this is logic, not guessing. You are using the current candidates to prove that a number must either go in one place, or cannot stay in another place.

In SudoSketch, this technique is designed to work with notes and Coach highlighting, so the key cells and removal cells can be shown clearly.

How Almost Locked Set works

Find a small group of cells with one more candidate than cells.
That group is almost locked because removing one candidate would lock the rest.
Use links between ALS groups to remove candidates outside the set.

When to look for it

Look for Almost Locked Set after easier moves have stopped working. First check for naked singles, hidden singles, locked candidates and simple pairs. If those do not move the puzzle forward, this technique may be worth checking.

For learning, do not try to scan for every advanced technique at once. Pick one method, understand the shape, and practise spotting that one pattern.

Common mistake

The ALS must have one more candidate than cells. Otherwise it is not almost locked.

If you are unsure, rebuild the candidates first. Bad notes create bad logic, and Sudoku will absolutely punish you for it like a tiny spreadsheet goblin.

Almost Locked Set FAQ

What is Almost Locked Set in Sudoku?