AppendixB Guide to Notation We use diagrams throughout the book to illustrate important ideas. Some diagrams are informal, like a screen shot of a dialog box or a schematic showing a tree of objects. But the design patterns in particular use more formal notations to denote relationships and interactions between classes and objects. This appendix describes these notations in-detail. We use three different diagrammatic notations: 1. A class diagram depicts classes, their structure, and the static relationships be- tween them. 2. An object diagram depicts a particular object structure at run-time. 3. An interaction diagram shows the flow of requests between objects.