Windoku 6×6 Hard — Set 51
Solver Notes
Windoku — Free Online & Printable
Windoku, also known as Window Sudoku or NRC Sudoku, adds four fixed 3×3 window regions to the standard 9×9 grid. The windows are positioned symmetrically: top-left at rows 2-4, columns 2-4; top-right at rows 2-4, columns 6-8; bottom-left at rows 6-8, columns 2-4; bottom-right at rows 6-8, columns 6-8. Every window must contain digits 1-9 without repetition, in addition to standard row, column, and box constraints. The overlapping nature of windows and standard boxes creates rich constraint chains that experienced solvers find deeply satisfying.
About the 6×6 Grid
The 6×6 grid strikes a balance between accessibility and depth. With 36 cells in 2×3 boxes, puzzles take 3 to 10 minutes depending on difficulty. The 6×6 is large enough to showcase variant rules meaningfully — Killer cages have room to create genuine arithmetic puzzles, Windoku windows overlap in interesting ways — while remaining completable in a single sitting. Many players find the 6×6 their "daily driver" format: challenging enough to be satisfying, short enough to fit into a break. Variant rules interact more visibly at this size than the 4×4.
Hard Difficulty
Hard difficulty introduces multi-step deductions where you must track constraint chains across multiple regions simultaneously. Required techniques include naked pairs and triples (two or three cells sharing the same possible values), pointing pairs (where a candidate is confined to one row or column within a region), and intersection removal. For Killer Hard, expect cage combination analysis where several possibilities exist and must be eliminated systematically. Average time: 20-40 minutes for 9×9. Most solvers begin using candidate notation at this level.
How to Get Started
- Choose "New Puzzle" to generate a fresh Windoku puzzle with a unique solution
- Click or tap cells to enter numbers (1–6)
- Use the constraint highlights to track which numbers are still available
- Print the puzzle if you prefer solving on paper