MeSH vocabulary is divided into four types of terms. The main ones are the "headings" (also known as MeSH headings or
descriptors MeSH terms are arranged in alphabetic order and in a hierarchical structure by subject categories with more specific terms arranged beneath broader terms. When we search for a MeSH term, the most specific MeSH terms are automatically included in the search. This is known as the
extended search or
explode of that MeSH term. This additional information and the hierarchical structure (see below) make the MeSH essentially a
thesaurus, rather than a plain subject headings list. The second type of term, MeSH
subheadings or
qualifiers (see below), can be used with MeSH terms to more completely describe a particular aspect of a subject, such as adverse, diagnostic or genetic effects. For example, the drug therapy of asthma is displayed as asthma/drug therapy. The remaining two types of term are those that describe the type of material that the article represents (
publication types), and
supplementary concept records (SCR) which describes substances such as chemical products and drugs that are not included in the headings (see below as "
Supplements").
Descriptor hierarchy The
descriptors or
subject headings are arranged in a hierarchy. A given descriptor may appear at several locations in the hierarchical tree. The tree locations carry systematic labels known as
tree numbers, and consequently one descriptor can carry several tree numbers. For example, the descriptor "Digestive System Neoplasms" has the tree numbers C06.301 and C04.588.274; C stands for Diseases, C06 for Digestive System Diseases and C06.301 for Digestive System Neoplasms; C04 for Neoplasms, C04.588 for Neoplasms By Site, and C04.588.274 also for Digestive System Neoplasms. The tree numbers of a given descriptor are subject to change as MeSH is updated. Every descriptor also carries a unique alphanumerical ID that will not change.
Descriptions Most subject headings come with a short description or definition. See the MeSH description for diabetes type 2 as an example. The explanatory text is written by the MeSH team based on their standard sources if not otherwise stated. References are mostly encyclopaedias and standard textbooks of the subject areas. References for specific statements in the descriptions are not given; instead, readers are referred to the bibliography.
Qualifiers In addition to the descriptor hierarchy, MeSH contains a small number of standard
qualifiers (also known as
subheadings), which can be added to descriptors to narrow down the topic. For example, "Measles" is a descriptor and "epidemiology" is a qualifier; "Measles/epidemiology" describes the subheading of epidemiological articles about Measles. The "epidemiology" qualifier can be added to all other disease descriptors. Not all descriptor/qualifier combinations are allowed since some of them may be meaningless. In all there are 83 different qualifiers.
Supplements In addition to the descriptors, MeSH also contains some 318,000
supplementary concept records. These do not belong to the controlled vocabulary as such; instead they enlarge the thesaurus and contain links to the closest fitting descriptor to be used in a MEDLINE search. Many of these records describe chemical substances. ==Use in Medline/PubMed==