What is "hatris"

is a puzzle video game developed by Alexey Pajitnov for Bullet-Proof Software. It was also released as an arcade game.

The game plays similarly to Pajitnov's previous Tetris, in that game objects falling from the top of the screen must be arranged in specific patterns to gain points and to keep the play area clear. In Hatris, hats of different styles fall from the top of the screen and accumulate at the bottom. To eliminate hats from the play area, five hats of identical style must be stacked. Different style hats stack differently.