Chapter 12. Modules and Mixins

In Ruby, each class has only one immediate “parent,” though each parent class may have many “children.” By restricting class hierarchies to a single line of descent, Ruby avoids some of the problems that may occur in those programming languages (such as C++) that permit multiple lines of descent. When classes have many parents as well as many children and when their parents and children also have other parents and children, you risk ending up with an impenetrable network (a knotwork?) rather than the neat, well-ordered hierarchy that you may have intended.