ALS chains: pass a restriction through several sets

An ALS chain joins multiple almost locked sets. Consecutive sets use restricted common candidates to pass a forced alternative from one end to the other.

Key idea

If an outside candidate sees the end occurrence of the same digit at both ends, that candidate can be removed.

See the move, step by step

Follow a three-set chain through candidates 6 and 5 to eliminate 7.

Step 1 of 4

Choose the chain's end sets

Start with small ALSs in row 1 and column 7, then check whether a middle set can connect them.

Chain units

Step 2 of 4

Link consecutive restricted candidates

The first and second sets connect on 6; the second and third connect on 5. The alternating links carry the implication.

Chain unitsThree linked ALSs

Step 3 of 4

Compare the two ends

Candidate 7 is forced at one end or the other, and R3C1 sees both end occurrences.

ALS chainCandidate to remove

Step 4 of 4

Apply the endpoint elimination

Remove 7 from R3C1 without resolving the internal ALS chain.

ALS chainUpdated target

Try it on today's puzzle

Build chains one restricted common candidate at a time. Each adjacent ALS pair must satisfy the visibility rule independently.

Play today