FAQ

It's usually one of these:

• It isn't your turn yet: the computer is still thinking.
• You're looking back at an earlier position in the move list. Press "Back to the game" and you can move again.
• Your king is in check, so you can only play moves that get out of check.
• The piece is pinned: moving it would expose your own king, so it can't move.
• The game is already over.

Castling has four conditions. If the king or rook has moved, if a piece is in between, if you're in check, or if the king would pass over an attacked square, castling isn't allowed. See Special moves.

Not against the computer. You can take back moves even after you lose or resign.

Yes. Games against the computer aren't saved: if you refresh or close the page, you go back to the screen where you choose the level. The level and colour you picked are remembered on this device.

No. Just open it in the browser on your phone, tablet or computer.

The AI for Beginner to Advanced has to download its model into your browser first. After that, it doesn't download again. Until it's ready, a simple practice opponent plays for it. See Computer opponents and levels.

Each move is written as "piece + the square it moved to". The piece is shown as a small icon, and pawns have no icon. Squares are named by the file letter and rank number, for example e4. Other symbols:

It adds the file letter or rank number of the square the piece came from, right after the piece icon. For example, if both knights can reach d2, the knight icon is followed by bd2 or fd2, meaning the knight from the b-file or the f-file.

For the full rules of notation, see Reading chess notation.