Play a friend

What the link actually does

It carries a code like g0 or m1f: one letter for the difficulty, then the number of the puzzle. Open it and the board is built from that code, so two people on opposite sides of the world are working the identical grid rather than two of the same level.

That matters more than it sounds. Two Sudoku labelled "hard" can differ enormously in how long they take, so comparing times across different puzzles tells you nothing. Comparing them on one grid is the only version of the question worth asking.

The code is short enough to read down a phone or type by hand, and it never expires. The puzzle behind a code is fixed for as long as the site exists, because the library it points into is only ever added to, never reordered.

Both of you at once, on one board

Open the link at the same time as the other person and you are working one grid between you, not two copies of it. A digit typed on either side appears on the other as it is typed, pencil marks included, and each player's cursor sits in the square they are looking at in their own colour — so you can see somebody is about to write in the square you were thinking about.

Press With a friend on any play page and copy the link, or open a link somebody already sent you. The board joins its game as soon as that panel is open, and the line above the grid says how many of you are on it. If your colour is already taken you are quietly moved to a free one, and you can pick a different one whenever you like.

If the connection drops, or the sync service is unreachable, the grid carries on working exactly as it did before and the only thing missing is the other person. Nothing waits on the network before letting you write in a square, so a bad line slows their view of your board rather than your own typing.

Coming back to a game

A shared game keeps what has been written in it for a day after the last digit, so a grid left open at lunchtime is still there in the evening, exactly as the two of you left it. Nothing expires mid-puzzle and nobody has to be the one who stays online.

After that the shared copy is gone, and the link still opens the same puzzle — it always will — from whatever your own browser remembers of it. Progress is kept on the device you are using as well, which is why a link reopened on a phone weeks later is not an empty grid.

If you want a clean run at a grid you have already touched, the clear button empties the board without changing the link.

What is stored, and what is not

A shared game holds the digits and the pencil marks on that grid, a colour, and which square each player is in. That is the whole of it. There is no account, no email, no name and nothing that identifies you: two browsers holding the same code are the only thing that makes them the same game.

It follows that the link is the password. Anyone who has it can open the board and write in it, so send it to the person you meant to play and not to a public timeline. There is nothing to revoke, but the code belongs to the grid: start another puzzle and you are in a new game under a new code, with the old link left where it was.

Playing on your own touches none of this. A puzzle without a share code never contacts anything, and the ranked duels are a separate thing again, with a name attached because a rating has to belong to somebody.

Common questions

The other pages

• Printable SudokuGenerator
• Easy SudokuSingles only
• Medium SudokuNaked pairs
• Hard SudokuPointing pairs
• Expert SudokuBox/line reduction
• Printable SudokuOn paper
• Race a friendTwo boards, one race
• Ranked SudokuRated duels
• Sudoku ladderStandings
• Sudoku solverSolver
• Blank gridsPrintable
• Two grids a pagePrintable
• Large print gridPrintable