Cat Sudoku exclusion marks record a specific conclusion: “a cat cannot go here.” Plainoku uses manual crosses for these notes, helping you track the consequences of a placement without filling the board with numbers.
A useful cross has a reason behind it. The goal is to keep that reasoning visible, not to mark as many squares as possible. Start with the placement rules and use this workflow when a board has more possibilities than you can comfortably remember.
How do Cat Sudoku exclusion marks work?
Press Mark below the board to switch from placing cats to placing crosses. With Mark active, tap an empty square to cross it out. Tap a crossed square again in the same mode to clear the cross. Turn Mark off before placing the next cat. N also toggles the mode when using a keyboard.
A cross is not a small numbered candidate list. It means that the square is excluded by your current reasoning. Notes are manual: placing a cat does not automatically cross out its row or neighbors. Marking an occupied square replaces its cat with a cross; placing a cat on a crossed square replaces the cross. Check the highlighted mode before editing an important square.
Record four consequences of a placed cat
After a justified placement, review four sets of squares: the rest of its row, the rest of its column, the remaining squares in its color region, and its touching neighbors. Every square in these sets is unavailable to another cat while that placement remains.
Work through them in a consistent order. Mark the row, then the column, then the region, then the diagonal neighbors that were not already covered. Several reasons can exclude the same square; one cross is enough. Keeping the order consistent helps you notice a missed corner or a distant part of an irregular region.
A small coordinate example for your notes
Suppose a cat belongs at row 3, column 3 on a 5×5 board. Cross out the other positions in row 3 and column 3. Also exclude any other squares belonging to that cat's region. The adjacent diagonal squares R2C2, R2C4, R4C2 and R4C4 are excluded because they touch its corners.
R1C1 is farther away along a diagonal. Do not cross it out just for being on that diagonal: it does not touch R3C3. It may have another exclusion reason, such as belonging to the same region, but that reason must be checked separately. These coordinates illustrate the rules; they do not assert an answer for whichever puzzle you have opened.
Scan remaining candidates after updating marks
Choose an unfilled row and inspect its remaining legal positions. Then inspect an unfilled column and region. “Uncrossed” is a useful starting point, but still compare a square with the placed cats. A missing cross does not make a conflicting square legal.
If a unit has only one legal position, its cat is forced there. Place that cat, record the new exclusions, and scan again. If a region's remaining candidates all lie in one row, that region must use the row; other regions cannot put their cats there. The scanning guide describes this shared-row reasoning, while the forced-square guide explains why checking columns can reveal a position missed in a region.
Separate deductions from tentative choices
Crosses based on a certain placement and crosses based on a tentative placement look the same. Before trying a possibility, make a mental note of which cat you are testing. Avoid adding a long sequence of speculative crosses without checking what each one depends on.
A red warning flags conflicts between cats, not incorrect notes. A wrong cross can hide a valid square without turning red. When every position in a unit appears excluded, check the reasons for those crosses before assuming the puzzle is impossible. Review the most recent assumption first, rather than restarting a board whose earlier deductions remain useful.
Correct notes with Undo, Erase and the active mode
Undo restores the previous arrangement of cats and marks. If you remove an earlier cat directly, its dependent crosses are not automatically removed. Revisit those squares: a cross supported only by that cat should be cleared, while a square still excluded by another cat can remain marked.
Erase clears the selected square, whether it contains a cat or a cross. Keyboard focus selects a square without making a move; Delete or Backspace then clears it. Z undoes a move. On touch screens, tapping also uses the active mode, so clearing a cross with a second tap in Mark mode is often the simplest correction.
Use Hint without losing track of your reasoning
Hint can remove an incorrectly placed cat, or add a cat at one correct position and clear a cross from that square. It does not rewrite all your notes or explain the deduction. Inspect the changed position, then review the exclusions affected by it.
Your current cats and crosses are saved in this browser when storage is available. Undo history is not the same as saved progress and can reset after reloading. After returning to a puzzle, make a fresh scan before trusting a note you no longer remember. Completion depends on the cats satisfying the rules, not on crossing out every empty square.
Practise exclusion marks on a 5×5 Cat Sudoku board, or use the step-by-step controls guide to start a new puzzle.