MeSH descriptors are organized into 16 categories represented by letters: from A to N, V ​​and Z. DeCS descriptors are organized into 5 categories represented by two letters: HP, MT, SH, SP and VS. Within each subcategory, the descriptors are hierarchically organized from the most general to the most specific in up to thirteen hierarchical levels. Due to the branching structure, hierarchies are sometimes called « trees ». Since the thesaurus is polyhierarchical, each DeCS/MeSH descriptor appears in at least one location in the branches, and may appear in as many additional locations as appropriate.

In the tree view, each descriptor is followed by a code that serve only to locate the descriptors in each tree, having no intrinsic meaning. Codes are subject to change when new descriptors are added or when the hierarchy is revised annually to reflect vocabulary changes.

See the descriptors tree and the qualifiers hierarchical structures.

 Updated: June, 2022