Sudoku

Every row, every column and every box: one to nine, once each

SteadySingles all the way, but you will have to hunt for them.

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Sudoku

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The puzzle

Level

Keep the pencil marksEvery empty square shows what can still go in it, kept up to date as you write. Tell me straight awayA digit that isn't the answer turns red the moment you write it, and counts against you. Light up the same digitPick a square and every other square holding that digit lights up with it.

Play

Mode

How to play

Fill the grid so that every row, every column and every three-by-three box holds one to nine, once each. The digits printed in dark are the clues; they never move.
Pick a square and type a digit, or tap one on the pad below the grid. ⌫ or 0 clears it, and the arrow keys walk about the grid.
Pencil marks are the small digits: press Space (or the Pencil button) and what you type goes in small. Tick Keep the pencil marks and the page works them out for you and rubs them out as you write digits in.
A digit that repeats in a row, column or box turns its square pink — that much you could see for yourself. Whether it is the right digit is another matter: Check tells you, or tick Tell me straight away and it says so as you write.
Hint finds the next square that can be worked out from what is on the grid, says which rule does it, and fills it in if you ask again. It never guesses, because it never has to.
Keys: 1–9 write, Space pencil marks, H hint, C check, U undo, N new puzzle, M sound, F full screen.
Race a friend sends a link. You both get the very same puzzle and can watch each other fill it in; first one finished wins.

How to get through one

Last digit standing. If eight of the nine digits already appear in a square's row, column and box between them, the ninth goes in. This is the one everybody finds first.
Only place it fits. Take a digit and a box, and cross off every square in that box that its row or column rules out. One square left is where it goes — even when that square could hold other digits too.
Pointing pairs and box lines. If every place a 6 could go in one box sits in the same row, then a 6 is somewhere in that row inside that box — so the rest of the row is free of 6s. The same works the other way round, from a row into a box.
Pairs and triples. Two squares in a row that can only be a 4 or a 9 have the 4 and the 9 between them, whichever way round; no other square in that row can take either. Three squares sharing three digits do the same.
Hidden pairs. If only two squares in a box can take a 3 or an 8, those two squares hold the 3 and the 8 and nothing else, however many other digits they looked like they could take.
X-wings and XY-wings come last, and only Fiendish puzzles need them. Ask for a hint on one and it will spell out the chain.

About the puzzles

Each one is made fresh on your device: a grid is filled at random, then clues are taken away for as long as exactly one answer remains and the solver can still reason its way to the end.
You will never have to guess. Every puzzle here is finished by the rules above alone — the solver that grades it is the same one behind the Hint button, so if it got through, so can you.
The level is how far up that list of rules you have to climb, not how few clues are left. Gentle and Steady need only singles, Tricky needs a row or a box read as a whole, Fiendish needs a pair, a triple or a wing.
Clues come away in twos about the centre, which is why the grid has the symmetry a printed sudoku has. Tricky and Fiendish break it for the last few clues — puzzles that need real technique are hard to find otherwise.