What are locked candidates?
Locked candidates occur when the positions of a digit are constrained by both a 3×3 block and a row or column. You do not yet know the exact cell containing the digit, but you know it must lie in the intersection of those two regions. That is enough information to eliminate the same candidate from other cells.
This technique is powerful because you do not need to choose a specific cell. You use the relative positions of a candidate to narrow the board, then wait for another basic deduction to appear.
From a block pointing to a row or column
If candidate 7 remains in only two cells of a 3×3 block and both cells are on the same row, the block's 7 must be on that row. Therefore every other candidate 7 on the same row but outside the block can be eliminated.
The key condition is that every remaining position for the candidate in the block must lie on the same row or the same column. If the block still has a third position on another row, you do not have a pointing pair/triple.
- Start with one block and choose a candidate.
- Check every remaining position for the candidate in the block.
- Only eliminate candidates from the row/column outside the block.
From a row or column locked into a block
Claiming works in the opposite direction. If candidate 4 in a row can only appear in two or three cells and all of them lie in the same block, then the row's 4 must be in that block. You can eliminate candidate 4 from other cells in the same block that are not in the row.
Pointing and claiming are the same intersection idea viewed from opposite directions. Learning both under the single concept of “locked candidates” helps you recognize the pattern faster instead of memorizing two separate tricks.
Three questions to ask before eliminating a candidate
Before removing a candidate, check: have you listed every position for the digit in the source region; do those positions really share one row/column or block; and is the eliminated cell in the correct remainder of the target region? These three questions prevent most technique errors.
After an elimination, rescan the changed region for a naked single, hidden single, or new pair. Locked candidates often do not place a digit immediately, but they create the conditions for another technique to work.
Apply it immediately on a real puzzle
A technique becomes automatic only after you use it across many different boards. Play at a manageable level, prioritize certain deductions, and review the step that caused you to get stuck.