# Almost Locked Sets (ALS)

An Almost Locked Set is a group of N cells in a single house with candidates for N+1 digits. In other words, it is one cell short from being a locked set.

The simplest ALS is a single cell with 2 candidates. In a standard Sudoku an ALS can have a maximum size of 8 cells with 9 digits.

It is not easy to spot a large ALS, but there are often a lot of these sets present in the grid. Bivalue cells are always easy to spot with pencilmarks.

### Usage

There are several solving techniques that make use of Almost Locked Sets. Any move that would eliminate all candidates for a particular digit from the ALS automatically forces the remaining digits to be locked in the group of cells. This the equivalent of a strong link, which can be used in chains and loops.

The ALS-XZ technique uses only 2 Almost Locked Sets to perform eliminations. It is the most basic ALS technique.

### Solving Sudoku Strategy - Almost Locked Sets

ALS-XZ rule
2 Almost Locked Sets with restricted common digit X perform eliminations for common digit Z.
ALS-XY-Wing rule
3 Almost Locked Sets with 2 restricted common digits Y and Z perform eliminations for common digit X.
Death Blossom
A stem cell of N candidates pointing to N petals, each an Almost Locked Set.
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