Almost locked sets: one candidate away from a locked set
An almost locked set is N cells in one unit containing exactly N+1 distinct candidates. Removing any one candidate locks the remaining digits into those cells.
Key idea
An ALS is usually a building block rather than a move by itself. Seeing its restricted alternatives makes advanced links possible.
See the move, step by step
Start with two cells containing the three digits 5, 7, and 8.
Step 1 of 4
Search a unit for a compact candidate union
In row 1, compare small groups of unsolved cells rather than isolated notes.
Step 2 of 4
Count cells and candidates
R1C1 is 5/8 and R1C2 is 7/8: two cells with three total candidates form an ALS.
Step 3 of 4
See what one removed candidate would do
If 8 is supplied by R2C2, both 8s disappear from the ALS and its cells lock to 5 and 7.
Step 4 of 4
Read the resulting locked set
R1C1 becomes 5 and R1C2 becomes 7. The example shows why N+1 candidates are only one step from being locked.
Try it on today's puzzle
Practice identifying ALS shapes before trying ALS-XZ or ALS chains. Count the distinct candidate union, not the number of pencil marks.
Play today