Locked Candidates
When a digit's candidates inside one unit fall entirely within a second unit, eliminate that digit from the rest of the second unit.
Intermediate sudoku technique
What it is
Locked Candidates is a single principle with two manifestations. Type 1 — Pointing (box → line). A digit's candidate cells inside a 3×3 box all sit in the same row or column. The digit must end up in one of those cells, so it can be eliminated from the rest of that row or column outside the box. Type 2 — Claiming (line → box). A digit's candidate cells inside a row or column all sit in the same box. The digit must end up in one of those cells, so it can be eliminated from the rest of that box. The two forms are mirror images. Both are standard medium-tier workhorses — they unlock more candidates than any technique beyond naked singles and hidden singles.
When to use it
After naked singles and hidden singles dry up. Scan digit by digit through each box (for pointing) and through each row and column (for claiming).
Worked example
Pointing example: in box 1, digit 5's only candidate cells are R1C2 and R1C3 — both in row 1. The 5 in box 1 must land in row 1, so 5 can be eliminated from R1C4, R1C5, R1C6, R1C7, R1C8, R1C9. Claiming example: in row 4, digit 8's only candidate cells are R4C7 and R4C8 — both in box 6. The 8 in row 4 must land in box 6, so 8 can be eliminated from R5C7, R5C8, R5C9, R6C7, R6C8, R6C9.
Try it
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