The annual authority list for the subject analysis of the biomedical literature in the National Library of Medicine (NLM). The list is divided into two sections, an alphabetical list of the subjects along with their reference code numbers (MeSH numbers), and a “tabular list,” called a “tree structure,” in which the subjects are categorized and subdivided, often to several levels, according to the hierarchical arrangement of the classification. Any document in the literature will be classified under as many subjects as are logically necessary. The volume is published by NLM but may only be obtained from the National Technical Information Service of the United States Department of Commerce.