Ranked Sudoku
You and one other player, the same grid, from the same moment. First to fill it in correctly wins the match and takes rating off the other. You need a name to play; a guest can play everything else on this site without one.
What happens when you press play
You join a queue, and the site looks for somebody rated near you. While it is looking it says so, and after half a minute it offers to stop being fussy and match you with whoever is waiting — which on a quiet evening is the difference between a game and a spinner.
When two players are paired they are both sent the same clues at the same instant, and nothing else. You each fill in your own copy. You cannot see their digits and they cannot see yours; what you see is how many squares they have filled and how many of those are actually right, which tells you where you stand without giving away any of the answer. There is no such line under your own name, deliberately: a running count of your own correct squares would tell you whether each digit was right as you typed it.
If the queue turns up nobody at all — and on a young ladder it will — the site puts you in against one of its own practice opponents rather than leaving you waiting. They are rated, they are on the ladder, and they are beatable: most people beat them most of the time.
Every duel is played at moderate difficulty. That is a deliberate flattening: a rating built out of easy wins and hard losses measures something nobody can name, and one level means every number on the ladder is comparable with every other.
Winning means the server checked it
A win is a completed grid that the server has compared against the answer, digit by digit. Your browser is never sent the solution while the match is running, which is the whole reason the result can be trusted: "I finished" is not a claim a player is able to make here, because the machine deciding it is not the machine you are typing on.
That also means the board is not a scratchpad you check yourself. A wrong digit is left alone rather than flagged — telling you a square is wrong during a race would be handing over the answer one square at a time. If you fill the grid and nothing happens, the grid is complete and not correct, and you are told exactly that and no more.
There is a floor on how fast a result is believed. A grid arriving faster than a person could read it is voided rather than awarded to either player, because awarding it to the opponent would create a better trick than the one being prevented.
Leaving, and coming back
Closing the tab does not lose the match, and it does not win one either. Your board is held on the server exactly as you left it, so a refresh, a tunnel or a flat battery costs you the time and nothing else — open the page again and the same grid comes back with your digits still in it and the clock still running.
Nor does your opponent leaving hand you the game. If they vanish you are told the seat is empty, and you still have to solve the puzzle to be credited with beating them. That is the opposite of how most ladders have handled a dropped connection, and the reasoning is that quitting only pays if it denies the other player the win — here it does not, so there is nothing to gain by it and no need to punish the person whose train went into a tunnel.
If neither player finishes, the session closes as a draw after half an hour and neither rating moves. Anybody who wants out sooner can resign, which counts as a loss, because a ladder where quitting is free is a ladder where every losing position is abandoned rather than played out.
What the number next to your name means
Everybody starts at 1000. After each rated match the winner takes points from the loser, and how many depends entirely on who was expected to win. Beat somebody rated well above you and you gain a great deal; beat somebody far below you and you gain almost nothing, because the result was never in doubt.
The same applies in reverse, which is the part worth understanding before you play somebody much stronger: losing to a far better player costs you very little. A mismatch is not a punishment for either side, and it is why matching you with whoever is available is a reasonable thing for the queue to do rather than a compromise.
New accounts move faster. For your first eighty games the rating swings hard, so it can find roughly the right level quickly instead of creeping towards it over a season. After that it settles down and each result is worth less, which is what makes a high rating mean something: it has to be held rather than hit once.
Playing the same opponent repeatedly is worth progressively less. Without that, two accounts could trade wins to inflate one of them, and the ladder would measure patience rather than Sudoku.
Common questions
Do I need an account to play Sudoku here?
Not for anything except ranked duels. Every puzzle, the solver and the printable sheets work with no account at all, and always will. A rating has to be attached to something durable, so a duel needs a name.
Can I see my opponent’s grid?
No, and neither can they see yours. You are shown how many squares they have filled and how many of those are right — enough to know where you stand, and none of the answer. Your own correct count is not shown, because watching it move as you type would tell you which of your digits are right.
Is there always somebody to play?
Effectively yes, which on a ladder this young means some of your opponents are the site’s own. If the queue finds nobody real within about three quarters of a minute, you are matched against a practice opponent rather than left waiting: a rated account, played by a program, aiming to be somewhat slower than a player of your rating. They are meant to be beaten more often than not, and they are on the ladder alongside everybody else. Every real player who turns up is matched first.
What happens if I lose my connection during a ranked game?
Nothing, until you run out of time. Your board is kept on the server and comes back when you reopen the page, and your opponent has to finish the puzzle to beat you rather than being handed the win.
Why is every ranked game the same difficulty?
So the ratings are comparable. If some matches were easy and others were stern, a rating would be an average across incomparable things and two players with the same number would not have shown the same ability.
Does losing to a much stronger player hurt my rating?
Barely. The rating change is based on how likely each result was, so losing a match you were expected to lose costs a point or two. Winning one you were expected to lose is worth a great deal.