The special thing about associations in a thesaurus (as compared to associations found in a typical index or glossary) is that they are typed. This is important because it makes it possible not only to say that two terms are related, but also how or why they are related. It also makes it possible to group together terms that are associated in the same way, thus making navigation much easier. Commonly used association types like “broader term”, “narrower term”, “used for” and “related term” are defined in standards for thesauri such as [Z39.19], [ISO 5964] and [ISO 2788].