I feel I didn't express enough how important Piet Zwart is for the graphic design of the second half of the Twentieth-Century. What's important is that Zwart is a designer from the Netherlands who bridges the tradition between Dada's playful vitality and De Stijl's functionalism and formal beauty. What is interesting about Zwart is that he had no previous training in typography or printing.
I'd like to discuss this more in detail. Here is a cool site with Zwart's work.