The National Library of Medicine uses a controlled vocabulary to index journals articles, this controlled vocabulary is called MeSH (Medical Subject Headings).
Subject experts assign specific MeSH terms where applicable. MeSH index terms will help ensure that your search results will include articles about your topic, even when different words were used to describe the topic in the title and abstract.
Subheadings are arranged in logical hierarchical groupings (families). For example:
A search of the subheading therapeutic use in PubMed would automatically explode to retrieve citations indexed with the subheadings administration & dosage, adverse effects, and poisoning.
Not all subheadings are placed in these groupings (as they do not logically fit).
MeSH headings are organized in a "tree" with 16 main branches:
D. Chemicals and Drugs
E. Analytical, Diagnostic and Therapeutic Techniques and Equipment
F. Psychiatry and Psychology
G. Phenomena and Processes
H. Disciplines and Occupations
I. Anthropology, Education, Sociology and Social Phenomena
J. Technology, Industry, Agriculture
L. Information Science
M. Named Groups
N. Health Care
V. Publication Characteristics
Each branch has many levels of sub-branches, and each heading has a position in the hierarchy.
Some terms appear in more than one branch of the tree. For example:
The hierarchy allows a MEDLINE/PubMed search of a broader term to include the narrower terms in all branches automatically. This is known as "exploding." For example, a search of ear in PubMed would automatically explode to include records indexed with Ear, External; Ear, Middle; and Ear, Inner, as well as all narrower terms under each of these.