Windoku is a 9x9 Sudoku variant with four shaded 3x3 windows laid inside the grid, each of which must contain the digits 1 through 9 exactly once — on top of the usual row, column, and box rules. The windows are deliberately offset so that none of them lines up with a standard box, and that misalignment does something unexpected: it creates a fifth region that nobody draws on the board. Windoku is also known as Hyper Sudoku or Windmill Sudoku.
What is Windoku?
Windoku takes an ordinary Sudoku and adds four more regions to it. Visually the effect is a windowpane: four shaded 3x3 squares floating in the interior of the grid, separated from each other and from the border by a single line of unshaded cells. Each shaded square behaves exactly like a row, a column, or a box — it holds each of the nine digits once.
The placement is the whole point. Each window sits one cell in from the border and one cell away from the centre lines, so it straddles the corners of four different standard boxes rather than sitting inside one. A digit placed in a window is therefore constrained twice over, by its box and by its window, and those two constraints cover different sets of cells. Windoku puzzles usually ship with fewer givens than a Classic puzzle of the same difficulty, because the extra regions carry the slack.
Windoku Rules
- Standard Sudoku rules: Each row, each column, and each 3x3 box must contain the digits 1 through 9 exactly once.
- The four windows: Each of the four shaded 3x3 windows must also contain the digits 1 through 9 exactly once.
- Window positions: The windows occupy rows 2–4 and rows 6–8, crossed with columns 2–4 and columns 6–8. Row 1, row 5, row 9, column 1, column 5, and column 9 lie outside every window.
- Windows are not boxes: No window coincides with a standard 3x3 box. Each one overlaps four boxes, taking a corner from each.
How the Windows Change the Grid
A cell in a Windoku grid falls into one of two categories. If it is inside a window, it belongs to four regions: a row, a column, a box, and a window. If it is not, it belongs to the usual three. That imbalance is a solving tool in itself — windowed cells resolve faster, so they are where you look first when you need momentum.
The more interesting consequence is structural. Because the four windows fit so neatly around the centre lines, they force a region into existence that is never drawn:
The hidden fifth region
The nine cells where rows 1, 5 and 9 meet columns 1, 5 and 9 contain each digit exactly once. That is nine cells — the four grid corners, the four edge midpoints, and the dead centre — behaving as a full extra region even though nothing on the board says so.
Here is why it is true. Rows 2, 3 and 4 together contain each digit three times. Two of those three sit inside the two windows that occupy those rows, since each window holds every digit once. The third occurrence therefore has to be somewhere in rows 2–4 outside the windows, which means in column 1, column 5, or column 9. So the block of cells in rows 2–4 at columns 1, 5 and 9 holds each digit exactly once — it is a region too. The same argument applied to rows 6–8 gives another, and applied down the columns instead gives two more in rows 1, 5 and 9 at columns 2–4 and 6–8.
Now account for rows 1, 5 and 9 as a whole: they hold each digit three times. Two of those are used up by the two column-derived regions just described. The one that remains has to lie in columns 1, 5 or 9 — which is exactly the nine-cell set we started with. Each digit, exactly once.
Solving Strategies
1. Scan windows before boxes
A window is a 3x3 region like any other, so hidden singles and naked singles work in it unchanged. Because windows overlap four boxes, a digit that is already committed in three of those boxes is often pinned to a single window cell — a hidden single that no box-only scan would ever surface. Get into the habit of scanning thirty-two regions rather than the usual twenty-seven lines and boxes.
2. Exploit the window-box overlap
A window is offset by one cell from the boxes, so it splits its nine cells unevenly across the four boxes it touches: four cells in one, two in each of the next two, and a single cell in the fourth. That lopsided split is what makes the overlap useful. If a digit in a box is confined to the cells that box shares with a window, it is confined to the window as well, so it can be eliminated from the rest of the window — the Windoku version of box-line reduction. The reverse works too, and the one-cell corner is the sharpest case of all: whatever goes there is simultaneously the box's only contribution to that window.
3. Use the fifth region deliberately
Draw it mentally at the start of every puzzle: four corners, four edge midpoints, centre. Whenever one of those nine cells resolves, check the other eight. Corners in particular tend to be the last cells standing in a Windoku solve, and the fifth region is usually what cracks them.
4. Watch the unshaded lines
Row 5 and column 5 pass through the middle of the grid without touching a single window, and rows 1 and 9 and columns 1 and 9 hug the border the same way. These lines are the least constrained parts of a Windoku board and are where the puzzle hides its difficulty. Save them for after the windows have given up everything they have.
5. Keep the standard toolkit
Everything in our solving techniques library transfers directly — a naked pair inside a window is a naked pair, an X-wing is an X-wing. The windows do not add new techniques so much as add new places for the familiar ones to appear.
Mini Windoku (6x6)
Mini Windoku brings the window rule to a compact 6x6 grid using the digits 1 through 6 with 2x3 boxes. Instead of four windows there are two shaded windows, each holding the digits 1 to 6 exactly once, offset from the standard boxes in the same spirit as the 9x9 version.
Two windows on thirty-six cells is a high density of constraint, which makes Mini Windoku unusually forgiving: a digit placed almost anywhere immediately says something about a window, and windows on six digits resolve quickly. It is the right place to build the habit of scanning a shaded region as if it were a box. Mini Windoku is free to play; the 9x9 version is part of the premium collection.
Tips for Beginners
- Treat a window exactly like a box. Every scan you already know — crosshatching, hidden singles, naked pairs — applies to it without modification.
- Remember the count: thirty-two regions. Nine rows, nine columns, nine boxes, four windows, and the hidden fifth. Missing one is how a Windoku solve stalls.
- Windowed cells first. A cell inside a window is constrained by four regions instead of three, so it is the cheapest place to find a single.
- Learn the fifth region by shape, not by coordinates. Four corners, four edge midpoints, centre — a picture, not a list.
- Don't assume a window is a box. They are shaded 3x3 squares but they are offset by one cell, and mixing them up produces contradictions that are painful to unwind.
- Start on Mini Windoku. The free 6x6 grid with two windows teaches the overlap habit on a board you can hold in your head.
Windoku is the most quietly elegant variant in the collection. Adding four regions to a Sudoku sounds like a small change, but the offset placement makes them interlock with the boxes in a way that produces a whole extra region for free. Solvers who know about the hidden fifth region finish Windoku puzzles noticeably faster than those who don't — and it is entirely deducible from the four windows you can see.