ALS Chain

Level 12 · Black Belt

Strategy:

Almost Locked Sets

Requires:

ALS-XZ

Overview

ALS-Chain links multiple ALS groups through restricted common candidates. When ALS groups share candidates in specific ways, powerful eliminations become possible.

How It Works

  1. Identify multiple ALS groups in the puzzle.
  2. Find restricted common candidates (RCC) that link ALS groups.
  3. An RCC appears in both ALS groups but all instances see each other.
  4. Chain the logic to find eliminations.

Example

Starting position

Look for ALS Chain.

1

Start with the first ALS

Start with the first ALS in the chain.

2

ALS Chain of 5 sets

Find a chain of 5 Almost Locked Sets connected by restricted commons.

3

Links use restricted commons

Each link uses a different restricted common candidate: a digit that can be true in only one neighboring ALS.

4

The chain forces the removal

This ALS Chain forces alternation across the sets, producing the same removal at the end of the chain.

5

Eliminate 1 from 1

Eliminate 1 from 1 because of r1c7 and r4c9.

6

Result

Result after applying the technique.

Tips & Patterns

  • ALS-Chains require solid understanding of basic ALS.
  • Start with ALS-XZ (simplest chain), then progress to longer chains.
  • These are among the most powerful elimination techniques.