An example Battenburg puzzle: around the marked point the four cells alternate odd (pink) and even (yellow) - 5, 2, 4, 7.
Battenburg Sudoku marks certain points where four cells meet. At every marked point, those four cells must alternate odd and even like a chequerboard - just like the pink-and-yellow squares of a Battenberg cake. Diagonally opposite cells share a parity; side-by-side cells are opposite.
The rule: a 2×2 chequerboard of parity
Look at the four cells around a Battenburg mark. Two diagonally opposite cells are even, the other two are odd - so no two cells sharing an edge have the same parity. Below, 6 and 8 (even) sit on one diagonal, and 3 and 5 (odd) on the other.
Even on one diagonal (6, 8), odd on the other (3, 5) - a chequerboard.
A worked example, step by step
Suppose one corner of a Battenburg mark is already a 6.
Step 0. One corner is a 6 (even).
Step 1 - the diagonal partner is even. The cell diagonally opposite the 6 shares its parity, so it must be even too (2, 4, 6 or 8).
Step 2 - the side cells are odd. The two cells next to the 6 are the opposite parity, so both must be odd (1, 3, 5, 7 or 9). One placed digit fixes the parity of the whole 2×2.
Step 2. Diagonal partner even; the two side cells odd - the whole square's parity is set.
Why parity is powerful
Fixing parity halves the candidates in each of the four cells at a stroke: an "even" cell drops 1, 3, 5, 7 and 9, and an "odd" cell drops 2, 4, 6 and 8. Combined with the ordinary rules - and with any sum or comparison clues nearby - that often turns the 2×2 into a cluster of near-singles. Battenburg marks are small but they punch above their weight.
Solving strategy
- One digit sets the pattern. Any placed value in a marked 2×2 fixes the parity of all four cells.
- Prune by parity. Pencil only even digits into even cells and only odd into odd ones.
- Combine with other clues. Parity plus a sum or inequality nearby frequently pins exact digits.
- Feed the grid. Each digit placed behaves like a given for its row, column and box.
Common mistakes
- Same parity side by side. Edge-sharing cells must be opposite parities - never both even or both odd.
- Applying it to unmarked junctions. Only the marked meeting points carry the chequerboard rule.
- Forgetting 5 is odd. The middle digit trips people up - it belongs in odd cells.
- Ignoring ordinary Sudoku. The four cells still obey their rows, columns and boxes.
Can I play it here?
Not yet - Battenburg 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:
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Battenburg 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. Expert Sudoku leaves 25 of the 81 cells filled, where the beginner tools dry up almost entirely; Evil Sudoku leaves 23, with no easy moves at all. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.