When a document is indexed in such a way that the subject terms "birds" and "anatomy" are combined as subject and sub-heading (e.g., "birds--anatomy") it is a pre-coordinate index system.
The indexer or index builder has pre-determined the relationship between the two subjects.
In a post-coordinate index system, the document is assigned the terms "birds" and "anatomy," but no relationship is assigned to them. The user has the option of conducting a search that finds documents that include one or both of the terms.
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