Locked Candidates
Locked Candidates is the first technique that eliminates candidates rather than placing digits. It rests on a simple overlap: a box shares three cells with each row and column that crosses it. If every possible spot for a digit in a box lies on one such line, the digit is locked to that intersection — it must be placed there, so it cannot appear anywhere else on the line.
The pointing form works from box to line. Suppose in the top-left box the only cells that can hold 4 are r1c1 and r1c2. Wherever the box's 4 ends up, it sits in row 1 — so 4 can be removed from r1c4 through r1c9, the rest of row 1 outside the box. You haven't placed anything, but you may have unlocked singles elsewhere.
The claiming form runs the other way, from line to box. If the only cells for 8 in row 5 are r5c4 and r5c6, both inside the central box, then the box's 8 must be one of those two cells — so 8 comes off every other cell of that box, including cells in rows 4 and 6.
Reach for Locked Candidates the moment singles dry up. It is quick to scan, needs no pencil-mark bookkeeping beyond one digit at a time, and its eliminations frequently cascade straight back into Hidden and Naked Singles.
- Scan one digit at a time and look at each box-line intersection: are all the digit's spots in the box on one line, or all its spots on the line in one box?
- Two or three candidate cells in a straight mini-row or mini-column of a box is the visual signature.
- An elimination only matters if the line (or box) actually holds the digit elsewhere — check before moving on.