To make a move, first (left) click on a piece, and then on
its destination square.
To deselect a piece, (left) click again on the selected piece.
To castle, (left) click on the king and then on its destination square.
The rook moves automatically with it.
To go back to the original setup, double click with the right mouse button
on the board.
To go back one move, click once with the right mouse button.
The applet checks legality of the moves, including check. It follows the
chess moves precisely, except that it does not allow you to promote a piece to
something different from a queen, and:
The applet does not determine stalemate or mate, draw by
repetition, or draw by the 50 moves rule.