Even/Odd Sudoku: parity is another candidate filter

Read the marker key

Even/Odd adds parity restrictions to ordinary Sudoku. On 9×9, even digits are {2,4,6,8}; odd digits are {1,3,5,7,9}. Grandmaster Puzzles uses squares for even cells and circles for odd cells in its stated rules. Read your particular puzzle’s key rather than assuming a familiar shape or color always means the same thing.

Nikoli — Sudoku rules · Grandmaster Puzzles — Even/Odd rules

Our example: filter in two stages

Suppose ordinary units leave E5 with {2,3,6,9}, and its marker requires an even digit. Remove 3 and 9, leaving {2,6}. That is progress, but not a placement. If a confirmed 2 later appears in row E, remove 2 and place E5=6.

The marker did the first job; the ordinary row rule did the second. Skipping the intermediate pair would hide the extra evidence needed to establish the final answer.

Unmarked is not automatically opposite

When a puzzle marks only some cells, an unmarked cell may be even or odd. Only an explicit rule saying all qualifying cells are marked would let you use the absence of a marker. “Not marked even, therefore odd” otherwise invents an extra constraint.

Parity and sums

Even plus even and odd plus odd both give even totals. Even plus odd gives an odd total. This can check compatibility in a puzzle that also includes sums, but an even sum alone cannot tell you that both cells are even.

Sudoku-Y currently offers Classic and Killer. This variation is not yet a playable mode here. Always read the puzzle maker’s rules before applying a technique.

Try before looking

References and further reading

Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.

• Nikoli — Sudoku rules
• Grandmaster Puzzles — Even/Odd rules