GridMewWhen a region locks a line
Exclude candidates using the intersection of a region and a row or column.
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The cat can be uncertain but usefulCheck every remaining candidateLook for the consequenceOriginal exampleOriginal GridMew lesson · Updated 15 October 2026
The cat can be uncertain but useful
A region may have several candidates, all within one row. You do not know which of those cells holds its cat, but you do know that the row’s single cat must be somewhere inside the region. Cells elsewhere in that row are therefore unavailable.
The same reasoning works with a column. It also works in reverse: if every candidate of a row lies in one region, that region’s cat must use the shared part, so other cells of the region can be excluded.
Check every remaining candidate
The word every matters. A single surviving candidate outside the shared line breaks the lock. An × should be included in the argument only when a valid rule explains it. Do not infer a lock merely from the region’s shape or its largest colored area.
The example below is selected because its verified trace contains an actual lock. The before-step board includes all preceding placements and exclusions. The explanation lists the source group, destination group, premises and cells removed.
Look for the consequence
A lock normally removes candidates rather than placing a cat directly. After applying it, scan the affected row, column and region again. A single may now appear. This two-step pattern is often easier to see than searching for a forced cat immediately.
Some puzzles use a related neighborhood argument: a cell that touches every possible cat location in a group can be excluded. That is different from a line lock and the hint text names it separately.
A real step from gm-6-0002
Every candidate in row 2 lies in region B (row 2, column 1; row 2, column 2; row 2, column 3; row 2, column 4). Its cat must use that shared section. Exclude row 3, column 3; row 3, column 4; row 3, column 5 from the rest of region B.