In order to organize the more than 500 elements used in the TEI schema into a comprehensible system, but also to group them together by similarity of semantics, structure, or location and make modification and customization possible, most TEI elements are defined through classes rather than directly. Attributes commonly used together are also grouped into classes. Classes can contain other classes. Elements inherit the properties of the classes of which they are members (Wittern et al. 2009, 285).