GridMewSpot the only candidate
Find forced cats by checking the remaining cells in a row, column or region.
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A quota creates a forced placementRecount after each placementSeparate a single from an empty groupApply the ideaOriginal exampleCheck your understandingOriginal GridMew lesson · Updated 16 October 2026
Learning goal: Prove that a group has exactly one candidate by explaining why its other cells are unavailable.
A quota creates a forced placement
Every unfilled row, column and region must eventually contain one cat. If all but one of its cells are excluded by valid deductions, the remaining cell must hold that cat. This is a single-candidate step. You need a reason for every exclusion; an empty-looking region alone is not enough.
The relevant group may be a column even when you started by looking at a region. Scan all three kinds of group. In the original example below, the explanation identifies the group and exact remaining coordinate.
Recount after each placement
A new cat may remove candidates from several groups at once. Place it, update the row, column, region and neighboring cells, and scan again. A board that initially seemed open can produce several singles in succession.
Do not continue scanning from an outdated drawing. One candidate that was valid before the last cat may now touch it. Updating immediately makes the next forced placement easier to justify and reduces accidental guesses.
Separate a single from an empty group
One candidate is a useful deduction; zero candidates signals a contradiction or mistaken note. Do not place a cat in an excluded cell to fill the quota. Check the recent move or recover a candidate by undoing the note that removed it.
Fixed cats count as already satisfying their row, column and region. You do not need to find another candidate in a group that already has its cat. The worked steps skip these completed groups.
Apply the idea to gm-6-0001
r2c3 means row 2, column 3, counting from the top left.
The bottom row of gm-6-0001 is F E E E E E from left to right. The fixed cat at r5c4 already fills E. Consequently the five E cells in row 6 are unavailable, leaving r6c1 as the row’s only candidate.
The conclusion is stronger than “r6c1 looks promising.” A cat must occupy row 6, and every alternative belongs to a region that already has a cat. After placing r6c1, update column 1; do not look for a second cat in F or another cat in the bottom row.
A real step from gm-6-0001
row 6 has only one remaining candidate: row 6, column 1. Every other cell is excluded by earlier placements or deductions. Place the cat here.
Check your understanding
What would change if you accidentally crossed out r6c1 before requesting a hint?
Show the explanation
Your notes would make the row look empty, but would not change the original deduction. Restore the unsupported candidate. GridMew’s game hints do not treat arbitrary × notes as established proof.