How to solve Windoku

Seeing it. Pick a digit a window doesn't have yet and ask where it can go. In the diagram, window 2 has no 1. The 1 in column 6 rules out the window's column 6 cells, and the 1 in row 5, column 7 rules out its column 7 cells and, through box 6, row 4, column 8.

Using it. Only row 3, column 8 is left, so it's the window's 1. A window shares cells with four boxes, so a digit placed anywhere near it rules out several of its cells at once. That makes hidden singles in the windows more common than you'd expect.

Seeing it. A cell in a window has neighbours that aren't in its row, column or box, so it's often down to one candidate sooner than you'd expect. In the diagram, the green cell is row 4, column 4, the bottom right corner of window 1. Its row, column and box hold every digit except 2 and 8.

Using it. Now look round its window. Window 1 already has an 8, in row 2, column 2, so the green cell can't be 8 and must be 2. When you list the candidates for a cell in a window, always include the digits in that window.

Seeing it. Each window overlaps four boxes: four cells with one box, two with each of two others, and one with the fourth. Look for a digit whose only places in a window are inside one of those overlaps. In the diagram, window 3 can only take 7 in row 6, column 2 or row 6, column 3, and both are in box 4.

Using it. The window's 7 is one of those two cells, so box 4's 7 is too. Remove 7 from the rest of box 4. Here that's the green cell, row 4, column 2.

Seeing it. Now turn it round. Look for a box whose only places for a digit are all inside one window. In the diagram, box 9 can only take 9 in row 7, column 7, row 7, column 8 or row 8, column 7, the three open cells it shares with window 4.

Using it. Box 9's 9 is in window 4, so the window's 9 is too, and no other cell in the window can be 9. The green cell, row 6, column 6, loses its 9.

Seeing it. A window works like a box with rows and columns too. Look for a digit whose only places in a window are on one row or one column. In the diagram, window 4 can only take 5 in row 6, column 7 or row 7, column 7.

Using it. The window's 5 is in column 7, so column 7's 5 is too. Remove 5 from the rest of the column: here the green cell, row 9, column 7.

Seeing it. Naked pairs and triples work in a window exactly as they do in a box. In the diagram, row 7, column 2 and row 7, column 3, both in window 3, can only be 3 or 6.

Using it. Those two cells take the window's 3 and 6 between them, so no other cell in the window can be 3 or 6. The green cells, row 6, column 3 and row 6, column 4, lose their 3 and are left with 8 or 9.

Seeing it. Look for two digits that fit in only the same two cells of a window. In the diagram, window 3 still needs 2 and 3, and only the green cells, row 6, column 2 and row 7, column 2, can take either of them.

Using it. Those two cells must be 2 and 3, in some order, so every other candidate in them can go. Both lose 1, 6 and 9.

Seeing it. The four windows create five more groups that aren't shaded but must hold 1 to 9 too. Take columns 2 to 4: between them they hold 1 to 9 three times. Windows 1 and 3 cover two of those three sets, so the nine cells left over, in rows 1, 5 and 9, hold 1 to 9 once. The same is true of the cells in rows 1, 5 and 9 of columns 6 to 8, and of the cells in columns 1, 5 and 9 of rows 2 to 4 and of rows 6 to 8. The fifth is the nine cells where rows 1, 5 and 9 cross columns 1, 5 and 9.

Using it. Treat each hidden window like any other group. In the diagram, the purple cells and the green cell form the hidden window down columns 2 to 4, and it needs a 4. The 4s marked blue rule out every cell except row 5, column 4, so it's 4, even though that cell isn't in any shaded window.