Mirrored from Sudopedia, the Free Sudoku Reference Guide
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 candidate digits.
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.