Chess clock

Two players, one device. Press your half of the clock to end your turn and start your opponent's.

How the chess clock works

A chess clock is two countdown timers joined so that only one runs at a time. When you have made your move you press your side, which stops your time and starts your opponent's. A player whose time runs out loses the game, unless the opponent has no way left to checkmate, which makes it a draw.

Time controls, explained

5 + 3: Each player starts with 5 minutes and gets 3 seconds back after every move; 5 + 0 adds nothing. The presets run from bullet, the fastest, through blitz and rapid to classical.
Increment: Also called Fischer time. A fixed number of seconds is added to your clock after every move you make, so a quick move banks time for later.
Simple delay: Also called US delay. At the start of each move your clock waits for the delay before it counts down. A move made within the delay costs nothing, but you never gain time.
Bronstein delay: Your clock counts down at once, and after your move you get back the time you used, up to the delay. A move never ends with more time than it started with.
Second time control: Classical games often add time after a set number of moves, such as 30 more minutes after move 40. Each half counts its own moves and shows when the extra time is due.
Time odds: A different starting time for each player, so a stronger player can give a weaker one more time to think.

Keyboard shortcuts

Space ends the turn that is running. Left Shift or A presses the left clock, Right Shift or L the right one. P pauses and resumes, and Esc opens the settings.

On a phone or tablet

Lay it flat between you. Held upright, the far half turns to face your opponent. While a clock runs the screen stays on, in browsers that allow it, and Full screen leaves nothing on the screen but the clock.

Low time and the flag

A clock shows Low time below a tenth of its starting time, or below ten seconds when that is more, with one quiet beep. At zero it shows Out of time and both clocks stop.

How the time is kept

The clock reads your device's precise clock every time it draws instead of counting ticks, so it stays right when the tab is in the background. A game left part way, by a reload or a swipe back, comes back paused. Your last time control and settings are saved in this browser only.