The most common language of numbers, the decimal system, has not always been used universally. From a purely mathematical point of view,the decimal system has no inherent advantages over other possible systems; its popularity is due to historical and biological, not mathematical factors. In this book, S. V. Fomin discusses the origin, properties,and applications of various number systems, including the decimal, the binary, and the ternary. His presentation offers the student an introduction to mathematical abstraction and then, through its examples, shows him the abstractionat work.