A component has at most one parent. A component might have multiple children. Some components accept only certain types of components as children. Some must be a child of certain type of components. Some don't allow any child at all. For example, Listbox in XUL accepts Listcols and Listitem only. Refer to Javadoc or XUL tutorials for details.

A component without any parent is called a root component. A page might have multiple root components, which could be retrieved by the getRoots method.