Interesting Stuff: Surprisingly enough, Mondrian started out as an expressionist. While studying in Paris before WWI he was exposed to abstraction and he never looked back.
Around 1914, you would likely find him saying things like "... art is higher than reality and has no direct relation to reality..." and "... art should be above reality, otherwise it would have no value for man...". Geez, settle down Piet...
Eventually he would limit himself to vertical and horizontal lines and the colors of red, blue, yellow, black, white, and grey.
If he looks familiar, his style has been ripped off by the makers of the L'Oreal Studio Line of hair care products, among others.