An association is a relationship that exists between two classes.

An association is binary when it links two classes, ternary when it links three classes, etc.

Associations can be compared to links between index cards.

The following drawing provides a three-dimensional view of the situations a class diagram can store.

Peter and Mary are clients. Peter has made reservations numbers 312 and 329.

A class diagram should be able to store all situations in the context of the company.

Examples of associations: