Almost Locked Sets
Level 9 · Purple Belt
Overview
An Almost Locked Set (ALS) is a group of N cells containing N+1 candidates. If one candidate is removed, the remaining N candidates are locked into the N cells.
How It Works
- Find a group of N cells containing exactly N+1 different candidates.
- All cells must see each other (share a house).
- If any candidate is removed from outside, the remaining N candidates lock.
- Use this to chain eliminations.
Example
Find two Almost Locked Sets (ALS): r4c4, r4c5, r4c6 and r4c9 and r5c7, r5c8, r6c8 and r6c9.
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Tips & Patterns
- ALS is a building block for advanced chains and techniques.
- Start by identifying ALS groups, then look for how they interact.
- Common: two cells with 3 candidates, three cells with 4 candidates.