Play Windoku Online

Windoku: The Window Sudoku Variant with Four Extra Shaded Regions

Windoku — also known as Window Sudoku, Hyper Sudoku, or NRC Sudoku — is one of the most elegant and strategically rewarding Sudoku variants. It places four extra shaded 3×3 regions, called windows, symmetrically inside the standard 9×9 grid. Each window must contain the digits 1–9 exactly once, just like rows, columns, and standard boxes. With 31 constraint groups instead of the usual 27, Windoku offers a beautifully balanced blend of extra information and deeper deduction.

🤔 What Is Windoku?

The name Windoku is a portmanteau of "window" and "Sudoku." The puzzle uses the same 9×9 grid divided into nine standard 3×3 boxes, but adds four additional 3×3 window regions that overlap with (but are offset from) the standard boxes. These four windows are positioned at:

• Window 1: Rows 2–4, Columns 2–4
• Window 2: Rows 2–4, Columns 6–8
• Window 3: Rows 6–8, Columns 2–4
• Window 4: Rows 6–8, Columns 6–8

On our game board, window cells are highlighted with a subtle shaded purple background so you can always see which cells carry the extra constraint. The four windows create a visually striking pattern that looks like four window panes set into the grid — hence the name.

📋 Rules of Windoku

Windoku follows five clear rule sets:

1. Row rule — Every row must contain the digits 1–9 exactly once.
2. Column rule — Every column must contain the digits 1–9 exactly once.
3. Box rule — Every standard 3×3 box must contain the digits 1–9 exactly once.
4. Window rule — Each of the four shaded 3×3 window regions must contain the digits 1–9 exactly once.
5. Uniqueness — Every puzzle has exactly one valid solution reachable through pure logic.

⭐ Difficulty Levels Explained

Our Windoku game offers four difficulty levels, each designed to test different skill levels:

• Easy — Around 38 given digits. The extra window constraints make many cells directly solvable. Perfect for your first Windoku experience.
• Medium — Around 30 givens. You'll need to actively cross-reference windows with rows, columns, and boxes. A satisfying daily brain teaser.
• Hard — Around 25 givens. Requires combining window-based logic with advanced techniques like naked pairs and hidden singles.
• Expert — Around 21 givens. Demands deep mastery of both standard Sudoku strategies and window-specific tactics. Only for experienced solvers.

🧠 Essential Windoku Strategies

All standard Sudoku solving techniques apply in Windoku, but the four extra windows unlock powerful new strategies:

1. Window Scanning

Treat each window like an extra 3×3 box. Scan all four windows to identify which digits are placed and which are missing. Because windows overlap with standard boxes, you'll often discover cells that can't be solved with rows, columns, and boxes alone.

2. Window-Box Interaction

Where a window overlaps with a standard 3×3 box, powerful interactions occur. If a digit is missing from both the window and the overlapping box, and can only go on shared cells, you can make strong eliminations in both regions simultaneously.

3. Cross-Hatch Zones

The cells lying between two windows' row or column spans form "cross-hatch" zones. These cells are constrained by their row, column, box, and at least one window — making them among the earliest solvable cells on the board.

4. Naked Pairs in Windows

Naked pairs and triples techniques extend naturally to windows. If two cells in the same window share exactly two candidate digits, those digits can be eliminated from all other cells in the window — just as you would in a row, column, or box.

5. Hidden Singles in Windows

When a particular digit can only fit in one cell within a window, that's a hidden single. Because windows overlap with standard boxes, hidden singles in windows are surprisingly common and extremely valuable for making progress.

🆚 Windoku vs. Classic Sudoku

How does Windoku compare to regular Sudoku?

• Extra constraints: Classic Sudoku has 27 constraint groups (9 rows + 9 columns + 9 boxes). Windoku adds four windows for a total of 31.
• Solving feel: The extra windows provide more information, which can make certain cells easier to solve — but you must track four additional regions alongside everything else.
• Fewer valid grids: There are far fewer valid Windoku grids than standard Sudoku grids, making puzzle construction more challenging for designers.
• Visual appeal: The four highlighted window regions give the grid a distinctive, elegant look that's instantly recognisable.

🆚 Windoku vs. X Sudoku

Both Windoku and X Sudoku add extra constraints to the classic 9×9 grid, but in different ways:

• X Sudoku adds two diagonal lines — 18 extra constrained cells forming an X pattern across the grid.
• Windoku adds four 3×3 window blocks — 36 extra constrained cells forming a symmetrical window pattern.
• Windoku has more total constraint groups (31 vs. 29), making it arguably the most information-rich single-grid Sudoku variant.
• Both are excellent next steps from classic Sudoku for solvers seeking a fresh challenge.

📜 A Brief History of Windoku

The Windoku variant first gained widespread attention in 2005 when the Dutch newspaper NRC Handelsblad began publishing it daily alongside standard Sudoku puzzles. In the Netherlands the variant became known as NRC Sudoku. Around the same time, puzzle publishers in the United States and United Kingdom independently discovered the format and marketed it as Hyper Sudoku, while Japanese puzzle magazines adopted the name Windoku (from "window" + "Sudoku").

The variant gained further recognition through its inclusion in the World Puzzle Championship and various international puzzle competitions. Today Windoku is one of the most widely published Sudoku variants, appearing in newspapers, puzzle books, mobile apps, and online platforms around the world. Its elegant visual design and balanced difficulty make it a favourite among casual solvers and competitive puzzlers alike.

💪 Benefits of Playing Windoku

• Expands your solving toolkit — learning to think in overlapping regions adds a valuable dimension to your logical reasoning skills.
• Improves spatial awareness — tracking windows alongside rows, columns, and boxes strengthens visual-spatial thinking.
• Fresh challenge for Sudoku veterans — if classic Sudoku feels routine, Windoku injects new life without completely changing the core rules.
• Competition preparation — window-based puzzles appear frequently in international puzzle championships, so practising Windoku sharpens your competitive edge.
• Brain training — juggling 31 constraint groups simultaneously is an excellent workout for memory, concentration, and analytical thinking.

🎮 More Sudoku Variants to Explore

• Classic 9×9 Sudoku — The original puzzle. Master the fundamentals first.
• Hyper Sudoku — Same window concept, different name — try it for alternate puzzle sets.
• NRC Sudoku — The Dutch newspaper variant with the same four window regions.
• X Sudoku — Adds diagonal constraints for a different kind of extra challenge.
• Killer Sudoku — Cage sums replace given digits for an arithmetic twist.
• Jigsaw Sudoku — Irregular regions replace the standard 3×3 boxes.