Tetris Holding v. Xio
The 2012 ruling that drew the line between a game's free rules and its protected look.
Game rules are ideas, ideas are not copyrightable, and cloners have done nicely out of that. Xio Interactive released Mino for the iPhone in 2009 having read up on the law: it deliberately copied "only the rules". In practice it copied nearly everything — the shapes and colours of the pieces, the 20 × 10 playfield, the ghost piece showing where a block will land.
The New Jersey court held in 2012 that such close visual similarity was infringement. The rules were free, but there were countless ways to express them, and Xio had picked Tetris's way in every detail. The ruling made it harder to hide a straight copy behind the idea–expression distinction, though the mechanics themselves remain fair game.