Most articles are on very specific subjects, and MeSH headings alone are not sufficient to cover this specificity. Subheadings are used to describe the specific aspects of the MeSH heading that are pertinent to the article. The subheading that reflects the main point of the article is made IM, while others are NIM. Most MeSH terms, especially those from Categories A, B, C and D, will usually be indexed with one or more subheadings. Usually no more than three subheadings are needed, however the indexer can use more than three, if the article demands it. A main heading - subheading combination appears as the main heading followed by a forward slash (/) and then the subheading. Indexers will index surgical treatment of myocardial infarction as:
MYOCARDIAL INFARCTION / * surg
Currently, MeSH has a total of 83 subheadings. Subheadings can appear in three forms: as a full name, as a two-letter abbreviation (used for PubMed searching), or as an “indexing” abbreviation. For example, the subheading Growth & Development can be written as /Growth & Development (full name), as /GD (two-letter abbreviation), or as /growth (indexing abbreviation).
It is important to use the most specific subheading possible for the article. Some subheadings fall into "natural" groups, which can be used to avoid unnecessary depth of indexing. For example, the general subheading /therapy can be used to group the more specific subheadings /drug therapy, /radiotherapy, /surgery, /diet therapy.
Another example is the general subheading /diagnosis, which can be used to group the more specific subheadings /radiography, /radionuclide imaging, /ultrasonography
Indexers must not force a subheading. If a subheading does not clearly apply, do not use it. Avoid using redundant subheadings, especially with terms from Category G. For example, subheading /genet with GENES does not convey any additional information and is not necessary.