ABSTRACT
The mathematical notion of a relation, like that of a set, is a very general one. It is one of the key concepts of mathematics and examples of relations occur throughout the subject. Three special types of relation are particularly important: functions, equivalence relations and order relations. Functions are the subject of the next chapter; equivalence and order relations are considered later in this chapter. We begin, though, with a look at the general concept of a relation and various ways of visualizing relations.
