Ninegrid
Two to four players get the same number grid and race to finish it. Every row, every column and every three by three box holds one to nine, once each. The first to fill all eighty-one squares wins.
The rules
- Everybody races the same grid. It is chosen from the game's seed out of six hundred, and every seat gets an identical copy; the seed is published when the race is over so the grid can be checked.
- Tap a square, then tap a digit. On a keyboard, the number keys write and 0 or backspace clears. The digits the grid came with are printed in bold and cannot be changed.
- A digit already in that square's row, column or box is refused. Your own digit can be written over, and cleared, as often as you like.
- A digit that clashes with nothing is accepted even when it is wrong, and the site does not tell you. Finding that out is the puzzle; if you are stuck, something you wrote earlier is wrong.
- Your grid is your own. The strip under the board shows how full each seat's grid is and nothing else, because showing which squares somebody had filled would be showing you the answer.
- Every seat presses Ready; when the last one has, a three-second countdown runs and the race begins for everybody at once.
How a race ends
- The first seat to fill all eighty-one squares wins. Two seats finishing in the same millisecond share first place.
- Once somebody has finished, the others have one minute. A seat still going when it passes is placed by how many squares it had right, which is not the same as how many it had filled.
- Nobody may take longer than fifteen minutes, whatever else happens.
- A resignation puts that seat out where it stands, and the others race on: the winner's time is still the result.
- The site's own move deadline applies here as a dead-man's switch rather than a turn clock: nobody is waiting on your move, so it only matters if you stop playing altogether. A table where everybody has gone quiet is abandoned, and an abandoned game is not rated.
Worth knowing before you start
- The clock is the server's, not your browser's. A square is timed from the moment the site receives it, so a slow connection costs a little on every square, and it costs everybody the same on average.
- A computer can solve one of these in a thousandth of a second, and nothing here can prove you did not use one. What the site does is refuse a finish faster than any person could manage: over a hundred and twenty squares a minute is disqualified rather than believed, and the others' result stands.
- Pencil marks are notes. Long-press a square, or hold shift, and a digit becomes a small note in the corner; they live in your browser only, are never sent, and nobody else sees them.
- The hint, in a game against the bot, is a square the strongest solver can prove right now, and it says which rule proves it. On a grid that has already gone wrong it offers the digit to take back instead.
The seed chooses the grid and is published when the race ends. Every grid has exactly one solution, so a full grid with no clash in it is the answer: the site never stores one and never needs to.
Ninegrid terms
- A given
- One of the digits the grid came with, printed in bold. It is the same for every seat and cannot be changed.
- A clash
- A digit that is already in the square's row, column or box. The site refuses it and flashes the square it clashes with.
- A pencil mark
- A note to yourself in the corner of a square. Yours alone, in this browser, and never part of the game.