ALS Chain

Level 11 · Red Belt

Strategy:

Almost Locked Sets

Requires:

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

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

Example

Look for ALS Chain.

Sudoku grid showing the initial puzzle state for Look for ALS Chain.

1

ALS Chain

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

Sudoku grid illustrating step 1: ALS Chain — Find a chain of 5 Almost Locked Sets connected by restricted commons.

2

ALS Chain

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

Sudoku grid illustrating step 2: ALS Chain — Each link uses a different restricted common candidate: a digit that can be true in only one neighboring ALS.

3

ALS Chain

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

Sudoku grid illustrating step 3: ALS Chain — This ALS Chain forces alternation across the sets, producing the same removal at the end of the chain.

4

ALS Chain

Eliminate 3 from 3 because of r3c2.

Sudoku grid illustrating step 4: ALS Chain — Eliminate 3 from 3 because of r3c2.

5

Result after applying the technique.

Sudoku grid showing the 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.