Sudoku rules

Sudoku is a number puzzle on a 9×9 grid. Some of the digits are already written in; you fill in the rest. There is one rule, and no arithmetic at all: the digits could as well be letters or colours.

The grid

The grid has 81 cells in nine rows, nine columns and nine boxes, the 3×3 squares marked out by the heavier lines. Every cell belongs to one row, one column and one box.

A sudoku as it starts. The digits printed in are the givens, which cannot be changed. The middle box is shaded: it is one of the nine.

The digits already there are the givens. They are part of the puzzle: you cannot change or rub them out.

The one rule

Every row, every column and every box must hold each digit from 1 to 9 exactly once.

That is all. Nine cells in a row and nine digits to go in them means no digit can be missing from a row and none can appear twice, and the same goes for every column and every box.

A finished row: 1 to 9, each once, in some order. Every column and every box of a finished grid looks the same.

The puzzle is solved when every cell is filled and the rule holds everywhere. A good sudoku has exactly one way to do that, and every puzzle here does.

Filling in cells

Pick a cell, then a digit: tap the cell and then a digit on the pad, or click it and type. Pressing the digit a cell already holds rubs it out, and so does Erase (or Delete, Backspace or 0 on a keyboard).

When you pick a cell, its row, column and box are lightly shaded, and every other cell holding the same digit a little darker. That is only a help for reading the grid: it knows nothing about the answer.

Beside each digit on the pad is how many of it are still to go. When all nine are down, the digit fades.

Undo (or Ctrl+Z, ⌘Z on a Mac) takes back your last move, and the one before that, as far back as you like.

Pencil notes

Hard puzzles are solved by writing down what a cell could be. Switch the pad from Pen to Pencil (or press N, or hold Shift while you type a digit) and the digits you press go into the cell as small notes, each in its own corner. Press a note again to rub it out. A note written over a digit takes its place; Undo brings the digit back.

One box with its candidates pencilled in: the box still needs 1, 2, 4 and 7, and each empty cell lists the ones it could be.

Notes are only for you and do not count for anything. When you write a digit in pen, the same note is rubbed out of every cell in its row, column and box, since it cannot go there any more.

Clashes

If a digit appears twice in a row, a column or a box, both are written in red. That is a mistake you can see on the grid itself, and pointing it out gives nothing away.

Two 5s in one row: both are marked in red until one of them goes.

Nothing else tells you a digit is wrong. A digit that clashes with nothing can still be the wrong one, and you will find out later, when some cell has no digit left that fits. That is the classic way to play, and it is what makes a solved sudoku yours.

The four levels

Every puzzle here is dealt fresh, and checked before you see it: it has exactly one answer, and you can reach it by logic, without guessing. The level says which techniques you will need, all of them explained in the strategy guide:

Level	What it takes
Easy	Hidden and naked singles only: every step is "this is the only place" or "this is the only digit".
Medium	Adds pointing, claiming and pairs, where pencil notes start to pay.
Hard	Adds triples, the X-wing and the XY-wing.
Expert	Adds the swordfish, the XYZ-wing and simple colouring.

A puzzle is given its level by solving it the way a person would, always with the simplest technique that makes progress; its level is the hardest one that could not be done without.

The clock starts with your first digit. If you leave in the middle, the puzzle is kept in your browser and comes back when you open the page again.

If you give up, Show the answer ends the puzzle and fills it in: cells you left empty get their digit in pencil, and digits you got wrong are written over in red.

Racing friends

Send the link to a table to up to three friends and you race each other on the same puzzle, two, three or four of you.

You each solve it on your own sheet, at the same time. There are no turns.
Whoever opened the table picks the level. Before the start, every friend who has sat down presses Ready, and whoever opened the table sees it and presses Start (at least one friend has to be seated). The race is dealt for everyone seated at that moment; anyone who arrives later watches. The digits appear for all of you at the same moment. Until then the grid is empty, so nobody gets a head start.
Fill in the whole grid correctly first and you win. The race ends there for everyone, and everyone else loses.
You see how much of each of the others' grids is filled in, as a share of the empty cells. Filled, not right: you never see which digits they wrote, and they never see yours.
Resign puts you out, and you have lost; the others race on, and whoever is left alone in the race wins. Someone who loses their connection has 60 seconds to come back; after that the others can race on without them, as if they had resigned.
Anyone still racing can offer a draw. It is a draw only when everyone still racing agrees; declining it, or carrying on writing before agreeing, turns it down.

After a race, Play again deals a new puzzle of the same level for the same players straight away, everyone moving one seat round, and keeps the score. Whoever opened the table can press Change settings instead, to pick another level first, and someone new can sit down then. The score is kept then too.

Playing on your own

On your own there is no computer on the other side: the puzzle is the opponent. Pick a level and start writing. The panel shows the time and how much of the grid you have filled.

Ready? Play sudoku, or read how to solve sudoku first. If you like logic puzzles with no guessing, try nonograms too.