This game is played with four combinators, represented by the letters I, K, S and Y.
You build five sequences of combinators at the right side of the playing field. Each time you add an extra combinator, the resulting combination will be reduced automatically according to the mathematics rules.

Combinators are taken from combinatory logic, which is a branch of mathematics.

Original web-game 'Combinatris' by Dirk van Deun
full description:
http://dirk.rave.org/combinatris/how-to-play.html