Facets are defined using a hierarchy of categories, known as a _Taxonomy_. For example, the taxonomy of a book store application might have the following structure:
Author
Mark Twain
J. K. Rowling
Date
2010
March
April
2009
The Taxonomy translates category-paths into interger identifiers (often termed ordinals) and vice versa. The category Author/Mark Twain adds two nodes to the taxonomy: Author and Author/Mark Twain, each is assigned a different ordinal. The taxonomy maintains the invariant that a node always has an ordinal that is < all its children.