Almost Locked Sets

Level 9 · Purple Belt

Strategy:

Almost Locked Sets

Requires:

Naked Pair

Learn after:

ALS-XZSue de Coq

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

  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.

Example

Look for Almost Locked Sets.

1

Almost Locked Sets

Find two Almost Locked Sets (ALS): r1c6 and r4c4, r4c6, r5c4, r5c6 and r6c4.

2

Almost Locked Sets

An ALS has N cells with N+1 candidates - eliminating any candidate locks the rest.

3

Almost Locked Sets

These ALSs share restricted common 9 (all instances see each other).

4

Almost Locked Sets

One ALS must contain the shared candidate 4, so eliminate 4 from: r6c6.

5

Result after applying the technique.

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.