How to Play Odd / Even Sudoku

Odd/Even Sudoku is a variant where some cells are shaded to indicate whether they must hold an odd or even digit. Purple-ringed cells must hold odd digits (1, 3, 5, 7, or 9). Yellow-shaded cells must hold even digits (2, 4, 6, or 8). Standard Sudoku rules apply to all cells.

Parity refers to whether a number is odd or even. In Odd/Even Sudoku, you can immediately halve the candidate list for any shaded cell. An odd cell can only hold 1, 3, 5, 7, or 9. An even cell can only hold 2, 4, 6, or 8. Combined with row, column, and box constraints, parity constraints make early placements much faster.

Shaded cells eliminate half the digit range immediately. If a row already contains three odd digits and has an odd-marked cell remaining, that cell is heavily constrained. Counting odd vs even cells in each row, column, and box often reveals hidden singles that would be invisible without the parity markings.

At easy and medium difficulty, it is comparable to or easier than classic Sudoku because parity markings provide immediate candidate reductions. Hard and expert Odd/Even puzzles use sparser markings and fewer given digits, making them comparable in difficulty to hard classic Sudoku.

Scan each row, column, and box and count how many odd and even slots remain versus how many odd and even digits are still unplaced. When only one cell in a row can hold an odd digit, that cell must contain an odd digit from the remaining candidates — this is a parity-based hidden single.

Easy puzzles take 5–15 minutes. Medium puzzles run 15–30 minutes. Hard puzzles average 30–60 minutes and expert puzzles can take 60–90 minutes. Parity-based hidden singles are the primary acceleration technique that distinguishes fast solvers.