  This text is aimed at undergraduates, or anyone else who enjoys thinking
about shapes and numbers. The goal is to encourage the student to think deeply
about seemingly simple things. The main objects of study are lines, squares,
and the effects of simple geometric motions on them. Much of the beauty of this
subject is explained through the text and the figures, and some of it is left
for the student to discover in the exercises. We want readers to "get their
hands dirty" by thinking about examples and working exercises, and to discover
the elegance and richness of this area of mathematics.
