Almost Locked Sets

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.

Technique

1. Find a group of N cells containing exactly N+1 different candidates.
2. All cells must see each other (share a house).
3. If any candidate is removed from outside, the remaining N candidates lock.
4. Use this to chain eliminations.

Tips

• 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.