Just as topics and occurrences can be grouped according to type (e.g., composer/opera/country and mention/article/commentary, respectively), so too can associations between topics be grouped according to their type. The association type for the relationships mentioned above are written_by, takes_place_in, born_in, is_in (or geographical containment), and influenced_by. As with most other constructs in the topic map standard, association types are themselves defined in terms of topics.