Locked Candidates in Sudoku: How to Spot Them

What is the Locked Candidates technique?

Locked Candidates appear when every possible position for one digit is trapped in the overlap between a 3×3 box and a row or column.

The lock tells you where the digit must eventually live. You may not know the exact cell yet, but you know enough to remove that digit from cells outside the locked area.

When to look for it

Look for Locked Candidates after you have reliable candidate notes and basic singles are no longer obvious. Scan one digit at a time inside a box, row, or column.

Two common forms are pointing and claiming. Pointing starts in a box and removes from the rest of a row or column. Claiming starts in a row or column and removes from the rest of a box.

Example walkthrough

This walkthrough uses a real expert app puzzle tagged Pointing Pair/Triple. In the right-middle box, candidate 4 appears only in the blue cells on column 9. That means the box’s 4 must be somewhere in that column inside the box, so r3c9 outside the box cannot keep candidate 4.

Why the elimination works

A 3×3 box needs exactly one 7. If all possible 7s in that box are on the same row, the box’s 7 will occupy that row. The rest of the row cannot contain another 7, so candidates outside the box can be removed.

Related concepts

Locked Candidates depend on accurate candidate notes. They often prepare the board for Naked Pairs and Hidden Singles because removing one candidate can reveal a smaller, easier pattern.

Common mistakes

Do not remove every candidate in the affected cells; remove only the locked digit. Also confirm there are no other possible positions for that digit in the starting box, row, or column before making the elimination.