An example Odd-Even puzzle: grey squares hold even digits (4, 6); circled cells hold odd digits (7, 3).
Odd-Even Sudoku shades or marks some cells to tell you the parity of the digit that belongs there. A cell drawn as a grey square must hold an even digit; a cell drawn as a circle must hold an odd one. Not every cell is marked, but the ones that are cut their candidate list in half before you place a thing.
The rule: squares are even, circles are odd
There are four even digits - 2, 4, 6 and 8 - and five odd ones - 1, 3, 5, 7 and 9. A square can only ever take one of the four evens; a circle only one of the five odds. Everything else is ordinary Sudoku: rows, columns and boxes still need 1-9 exactly once.
Grey squares hold even digits (6, 8); circles hold odd digits (3, 5).
The hidden count: four evens, five odds
Because there are exactly four even and five odd digits, every row, column and box contains precisely four even cells and five odd cells. When a whole unit is marked, that becomes a hard count: four squares and five circles, no more and no less. Even a partially marked unit gives it away - spot five circles and the four remaining cells must all be even, and vice versa.
A fully-marked box always has four squares (even) and five circles (odd) - the split is fixed.
Using parity as a filter
The real value of the marks is candidate pruning. A square instantly drops 1, 3, 5, 7 and 9; a circle drops 2, 4, 6 and 8. Combine that with a normal scan and hidden singles appear far sooner: if a box needs a 7 and only one of its cells is a circle, the 7 goes there without further thought. Parity also pairs beautifully with sum clues - in a marked cage, knowing which cells are odd or even often fixes the exact combination.
Solving strategy
- Prune first. Pencil only evens into squares and only odds into circles before any scanning.
- Count the parities. Four evens and five odds per unit - use it to place the last cell of a nearly-marked row or box.
- Target odd or even digits. When you need to place a specific digit, its parity tells you which marked cells can even hold it.
- Feed the grid. Each placement behaves like a given for its row, column and box.
Common mistakes
- Swapping the shapes. Squares are even, circles are odd - reversing them breaks every deduction.
- Assuming unmarked means anything. An unmarked cell has no parity clue, but it still obeys the four-even/five-odd count for its unit.
- Forgetting 5 is odd. The middle digit trips people up - it belongs in circles, not squares.
- Ignoring the count. The fixed 4-5 split per unit is a clue in its own right, not just a consequence.
Can I play it here?
Not yet - Odd-Even Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:
- Try the closely related X Sudoku, which you can play right now.
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Odd-Even Sudoku belongs to the constraint family and runs on 9×9. It is not one SudokuStreak generates yet - but the deduction it trains is the deduction every Sudoku runs on.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Hard Sudoku leaves 26 of the 81 cells filled, where scanning gets you started and then leaves you stranded; Expert Sudoku leaves 25, where the beginner tools dry up almost entirely. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.