Windoku, also called Hyper Sudoku, keeps every Classic rule and adds four shaded 3×3 windows, one cell in from each edge. Each window must also hold 1 to 9 once. The windows straddle the boxes, so a digit can be pinned down in places a Classic grid never allows.
The rules
Fill the 9×9 grid so that every row, every column and every 3×3 box holds the digits 1 to 9 exactly once, as in Classic Sudoku.
Four shaded windows sit on rows 2 to 4 and 6 to 8, columns 2 to 4 and 6 to 8. Each window is a house too: it must hold 1 to 9 exactly once.
Next Step names a window by where it sits, for example “the top-left window”, and lights it when it is the reason for a move.
What the windows change
A window does not line up with the boxes. Each one covers four cells of one box, two cells of two others and a single cell of a fourth, so a digit confined to that block of four is confined in two houses at once.
Cells inside a window see more of the grid. Outside the windows a cell sees 20 others, as in Classic; inside one it sees 23 or 24. More cells in view means fewer candidates in each window cell.
On this site a window decides at least one step in 75% of the puzzles, so the windows are part of the solve, not decoration.
How to solve it
Scan the windows first. Each is a fresh house of nine, and the one you are not used to looking at hides the easiest singles.
Look where a window overlaps a box in four cells. If the window’s only places for a digit are in that block, the rest of the box loses that digit, and the other way round.
There is a hidden extra: the four windows force five more groups of nine cells to hold 1 to 9 as well, such as the cells where rows 1, 5 and 9 cross columns 2 to 4. Strong solvers use them; every puzzle here can be solved without them.
Worked examples
Three positions from real puzzles on this site, walked through by the same Next Step teacher you get in a game. Step through each one.
A hidden single in a window
Look at the top-right window. The digit 6 must go somewhere in this window. Where can it go?
A window locks a digit into a box
In the top-left window, find every cell that can still take 1.
A row or box locks a digit into a window
In box 9, find every cell that can still take 5.
The four levels
Windoku levels use the same technique scale as Classic, so a Medium Windoku needs the same techniques as a Medium Classic puzzle. Every puzzle is solvable by logic alone, which is why there is no Diabolical level.
Classic techniques, with windows as extra houses
Every Classic technique works, and wherever a lesson says "box", a window counts too. These lessons teach them:
Frequently Asked Questions
Is Windoku harder than Classic?
Not by its levels: they use the same technique scale as Classic. The windows add houses, so they add information rather than take it away.
Where exactly are the windows?
Rows 2 to 4 and 6 to 8, crossed with columns 2 to 4 and 6 to 8: four 3×3 blocks, each one cell in from two edges, with a one-cell gap between them.
Do the boxes still count?
Yes. Rows, columns, boxes and windows all hold 1 to 9 once. The windows are added, nothing is taken away.
What are the hidden groups?
The four windows force five more groups of nine to hold 1 to 9: the cells of rows 1, 5 and 9 inside columns 2 to 4, and the same pattern elsewhere. Next Step does not use them, and no puzzle here needs them.
Why is it called Windoku?
The four shaded blocks look like the panes of a window. It is also known as Hyper Sudoku.
Why is there no Diabolical level?
Every Windoku on this site can be solved by logic, up to Extreme techniques. Diabolical puzzles need trial and error, so they are left out.
Which tools can I use?
All of them: pencil marks in corner or centre notation, candidates, Auto Check, Check Cell and Check Puzzle, and Next Step, which lights the window whenever it is the reason for a move.
Try one now. Easy is a good place to start.