A student in the primary or secondary schools frequently has the notion that mathematics is concerned solely with number and measure. However, mathematics has always been much more than merely a quantitative science with applications to activities such as bookkeeping and money-changing ; it is deeply concerned with logic and structure.

The theory of groups is one of the important non-quantitative branches of mathematics. The concept of a group, although comparatively recent in the development of mathematics, has been most fruitful; for example, it has been a powerful tool in the investigation of algebraic equations, of geometric transformations, and of problems in topology and number theory.

Two features of group theory have traditionally made it advisable to postpone its study until rather late in a student's mathematical education. First, a high degree of abstractness is inherent in group theoretical ideas, and ability to cope with abstract concepts comes with mathematical maturity. Second, the ways in which group theory interacts with other fields of study to illuminate and advance them can be seen only after long and elaborate development of the theory, and then only by students acquainted with the other fields. In this book we have aimed at a presentation suitable for students at a relatively early stage of mathematical growth. To bypass the difficulties stemming from abstractness, we have used geometric pictures of groups—graphs of groups. In this way, abstract groups are made concrete in visual patterns that correspond to group structure.