Well, square, rectagle, circular, etc. Really, it's just pretty geometric. The modular system comes with 25 pieces you can mix and match however you see fit and each piece is designed for a different purpose. Designers Erik Olovsson and Kyuhyung Cho based the shapes in each block so they could provide a home for specific objects -a triangle to house an open book, for example.
The end result is a cool-looking shelving system that is totally customizable. See more pictures of the ROOM Collection at Homes and Hues: A Delightful Geometric Modular Shelving System