There is no "exit" button, and the app eats the system buttons at the bottom of the screen so there's no way out of the app without serious lateral thinking. The most malicious UI pattern I've seen in a very long time. This is the reason I dropped from three stars to two. The AI needs work in the endgame (it has no strategy but to slavishly chase check at every opportunity, which is frustratingly obstructive), but up until then the game is quite fun. The ranking system is entertaining.