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 consequenceApply the ideaOriginal exampleCheck your understandingOriginal GridMew lesson · Updated 16 October 2026
Learning goal: Use every surviving candidate in a group to reserve an intersecting group without guessing a location.
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.
Apply the idea to gm-6-0002
r2c3 means row 2, column 3, counting from the top left.
After the opening row-2 lock in gm-6-0002, row 3 is left with r3c1 and r3c2. Both cells belong to C. Row 3 must have a cat, so C’s only cat must be in that row. The C cells in rows 4–6 can now be excluded.
This is the reverse of “a region is confined to a row.” Here a row is confined to a region. The quota argument works in both directions, but only if every candidate of the source group lies in the intersection. The following shared-neighbor exclusion is a separate step with a separate reason.
A real step from gm-6-0002
Every candidate in row 3 lies in region C (row 3, column 1; row 3, column 2). Its cat must use that shared section. Exclude row 4, column 1; row 4, column 2; row 5, column 1; row 5, column 2; row 6, column 1 from the rest of region C.
Check your understanding
Can C still take r5c1 while row 3 chooses between r3c1 and r3c2?
Show the explanation
No. Either row-3 choice would also fill C, giving that region two cats if r5c1 were used. You do not need to decide which of the row-3 cells wins to reject r5c1.