SudokuBoku

Hyper sudoku Windoku 9×9

Four shaded windows join the boxes as houses of their own — and every grid served here is tested to make sure ignoring them would sink you.

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Puzzle —
Rules
  • Fill every row, every column and every 3×3 box with the digits 1 to 9.
  • A digit can appear once in each of those — never twice.
  • There is exactly one solution, and logic alone will reach it.
  • Rows and columns each take the digits 1 to 9, once apiece.
  • The boxes are irregular shapes, drawn with the heavy outlines.
  • However a region bends, no digit repeats inside it.
  • The colour patches are cages; the small number is what a cage's digits add up to.
  • No digit repeats within a cage — nor in any row, column or 3×3 box.
  • There are no given digits: the sums alone pin down the whole grid.
  • Everything from classic applies: rows, columns and 3×3 boxes take 1 to 9.
  • The two tinted diagonals must each hold 1 to 9 exactly once as well.
  • All the classic constraints hold — rows, columns and 3×3 boxes take 1 to 9.
  • Each of the four shaded windows must also contain 1 to 9 exactly once.
  • The windows sit a step in from the corners, overlapping four boxes apiece.
  • Rows, columns and 3×3 boxes work exactly as they always have.
  • On top of that, two squares a chess knight's move apart may not hold the same digit.
  • Select a square and its knight partners are ringed with a dashed outline.
  • Every row, column and 3×3 box holds 1 to 9, exactly as in classic.
  • Along each grey path, digits strictly climb from the round bulb to the far end.
  • They don't have to be consecutive — each square just beats the one before it.
  • Rows, columns and 3×3 boxes take 1 to 9, exactly as in classic.
  • The digits along each arrow add up to the digit in the circle it comes from.
  • Digits may repeat along an arrow — as long as no row, column or box objects.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • Between squares in the same box you'll find a sign; its point aims at the smaller digit.
  • Every comparison inside every box is shown — a hundred and eight of them.
  • The higher levels hand you fewer digits, not fewer signs. Expert gives you four.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A white dot between two squares means their digits differ by one.
  • A black dot means one digit is exactly double the other. 1 and 2 are both, and always show black.
  • A boundary with no dot is a clue too — those two digits are neither one apart nor double and half. Every boundary is marked or deliberately bare.
  • Expert hands you no digits at all. The dots and the gaps are the whole puzzle.
  • Fill every row, column and 3×3 box with the digits 1 to 9, as usual.
  • A square with a ring behind it holds an odd digit — 1, 3, 5, 7 or 9.
  • A square with a frame behind it holds an even one — 2, 4, 6 or 8.
  • Most squares are marked with neither, and those are genuinely unrestricted.
  • Every square switches to the fully marked board — one level, gentler than it looks, and two puzzles sharing a grid rather than one.
  • Every row, column and box holds five odd digits and four even ones — so once five squares of a house are known odd, the rest of it is even, marked or not.
  • Solve the 9×9 sudoku as usual — every row, column and box takes 1 to 9.
  • Ten ships are hidden on the same squares: one of four, two of three, three of two and four single squares.
  • Ships are straight, and no two of them touch — not even at a corner.
  • The numbers beside and below say how many ship squares each row and column holds.
  • A ship listed with digits sits on exactly those digits, in order — so the grid is your map.
  • Five 9×9 grids interlock: four corners and a centre, each corner sharing one 3×3 box with it.
  • Every grid obeys ordinary sudoku on its own — rows, columns and boxes each take 1 to 9 once.
  • A tinted box belongs to two grids at once, and only together do the five have one solution.
  • Fill every row, every column and every box with the digits 1 to 6.
  • The boxes are three squares wide and two tall.
  • Nothing repeats in any row, column or box.
  • Put 1, 2, 3 and 4 into each row, each column and each 2×2 box.
  • No digit may appear twice in any of them.
  • Every square can be worked out — no guessing needed.
Keyboard shortcuts
1–9
Place a digit, or toggle a pencil mark
Arrows
Move around the grid
N
Notes mode on or off
Backspace
Clear the square
Ctrl + Z
Undo · Ctrl + Y redo

Windowed grids for the printer

A PDF of fresh hyper puzzles, built by your own browser. The windows print as a light grey wash under the digits — the same quiet signal the screen uses — and the answer key at the back repeats them, so a finished sheet can be checked against all thirty-one houses.

Difficulty

One rule, painted straight onto the grid

Hyper sudoku takes the classic board and shades four 3×3 squares — one sitting a single step in from each corner, their top-left cells at R2C2, R2C6, R6C2 and R6C6. Each shaded window must contain the digits 1 to 9 exactly once, on top of everything the rows, columns and boxes already demand. Thirty-six of the eighty-one squares answer to a window, and the count of houses rises from twenty-seven to thirty-one.

Unlike its cousin the X grid, whose extra houses hide in plain sight, this variant shows you exactly where the new pressure sits. That makes it kinder to start and stranger to master: the windows straddle four boxes apiece, so every deduction inside one drags four boxes, three rows and three columns along with it.

Three names, one puzzle

You'll meet this grid as hyper sudoku, as windoku — window sudoku — and as NRC sudoku, after the Dutch newspaper NRC Handelsblad, which began running it in 2005 and lent the variant its oldest name.

The five windows nobody paints

Here is the fact that makes this variant deeper than it looks. Take the top band of the grid — rows 2, 3 and 4. Those three rows hold every digit three times over, twenty-seven cells in all. Two painted windows sit wholly inside them, and each claims one copy of every digit. Subtract, and the nine cells left over — columns 1, 5 and 9, crossed with those same rows — must hold the third copy of each digit exactly once. That is a 3×3 all-different group the rules never mention.

Run the same subtraction over the bottom band, then sideways over the left and right stacks, and a fifth time on what remains, and you find the four painted windows secretly commit the grid to five more: the columns-and-rows left over between them behave as windows too. A hyper grid is really tiled by nine non-overlapping 3×3 all-different groups laid across the nine boxes — you were only ever shown four of them.

Paint four windows on a sudoku grid and you have silently ruled nine.

The four washed squares are the windows the rules name. The tinted cells are one of the five they imply: rows 2–4 hold each digit three times, the two washed windows there take one copy each, so the leftover columns must carry the third — once each, like a window of their own.

You can solve well without ever using this, and our hint engine deliberately plays by the four written windows only — a hint should never lean on a theorem the rules didn't state. But once you start seeing the hidden five, eliminations arrive from nowhere. The strangest of them is the last leftover group: the grid's four corners, its centre and the middle of each edge — nine cells that touch nothing shaded — can never repeat a digit among themselves. Write a 5 in the corner at R1C1 and the centre of the board has just lost its 5, though no row, column, box or painted window connects them. The strongest hyper solvers scan these scattered nine as routinely as any box.

What the windows do to the solve

Four more places a digit can hide

Every scanning habit you own gains new stops. Where can 7 still go in the top-left window? Nine cells, spread over four boxes — a shape your eye has never had to sweep on a classic board, wider than a box and squarer than a line. Digits with one home left in a window turn up constantly, because thirty-one houses thin the candidates faster than twenty-seven ever could. On this board those finds are announced by name, so the scan teaches itself.

The window interaction

The variant's signature elimination is a cousin of locked candidates, and here it runs in more directions than it does anywhere else on the site. A window shares three cells with each of six lines and up to four cells with a box. Corner a digit's last places within any of those overlaps and both houses pay for it at once: if row 2 can only put its 4 inside the top-left window, the window's 4 is spoken for, and every other cell of the window drops it — two rows down and a column across, in boxes the row never touches.

Row 2's last homes for a digit, tinted, all fall inside the top-left window — so the window's copy of that digit is pinned to that row, and the amber cells lose it. The same squeeze works from a box into a window, and from a window back out into either.

On an X board the equivalent move only ever trades between a diagonal and a box, because a diagonal crosses each line one cell at a time. Windows meet lines three cells at a time, so the interaction fires between windows and rows, windows and columns, and windows and boxes alike — which is why it carries the solve on our harder grids. One uniqueness-based technique goes the other way: the hint engine declines any unique rectangle whose corner sits in a window, since the window's own 1-to-9 promise can break the second solution that argument depends on.

Grids that prove their windows

There is a quiet failure mode in variant sudoku: a generator bolts the extra rule on, but the clues it leaves would pin down the same single solution under plain classic rules. The windows become decoration, and a solver who ignores them entirely loses nothing. We decided this page would be the first on the site to make that impossible by construction.

Every candidate puzzle is graded twice. Once under the full hyper rules, to find the techniques it demands. And once with the windows stripped away, counting how many solutions the bare classic grid would admit — if the answer is one, the windows carried nothing, and the puzzle is thrown away rather than served or kept as a fallback. The test bites hardest exactly where you'd least expect it: at generous clue counts. Leave forty givens on the board and over half of our candidate grids stay solvable without their windows; it took measuring that to settle Easy at thirty-four clues, where nine digs in ten land the right difficulty and nearly four in five genuinely need the glass.

Ignore the windows on this page's puzzles and yours will never be the only answer.

The other levels tighten from there — Medium digs to twenty-seven givens, Hard to twenty-three, Expert to twenty-two, each a couple below the equivalent classic targets because thirty-one houses squeeze a grid harder than twenty-seven. From Hard upward we additionally require the graded solve to fire the window interaction at least once, so the level that promises the variant's signature move measurably contains it. The ladder stops at Expert for the same reason the other variant pages stop there: our deepest levels rest on hand-verified banks that exist only for classic grids, and a label we can't measure is a label we won't print.

Playing it here, and taking it away

The windows wear the same sand wash the X board uses for its diagonals — one colour, one meaning across the site: these squares answer to an extra house. Select any washed square and its whole window lights alongside its row, column and box, which is the fastest way to grow the four-stop scanning habit. The share button hands anyone the same grid under the same rules — the link names the variant and the givens, the windows never move, and a shared game is saved separately so it can't trample one already underway here. And the pack builder above sends the wash to paper, where a printed booklet makes fine training for the moment a magazine springs windoku on you with no hint button in reach.