10×10, files a–j, ranks 1–10. Ten pawns a side. The back rank is R N C B Q K B C N R — the Checker (C) sits between knight and bishop on both wings. Squares are shaded so each queen starts on her own colour.
On its first move a pawn may advance one, two or three squares, provided every square along the way is empty. It can be captured en passant on any square it crossed, so a three-square dash cannot slip past a defending pawn.
The king still travels two squares toward the rook and the rook hops to the square it crossed: O-O puts the king on h1 and the rook on g1; O-O-O puts the king on d1 and the rook on e1.
Single player and White in hot seat use W A S D to move the cursor and Space to confirm. Black in hot seat uses the arrow keys and ,. Esc clears a selection, F flips the board.
Everything else is ordinary chess: promotion, the fifty-move rule, threefold repetition, stalemate, checkmate.