Comment on definition of 'Exp' in Table 1 Linda Slater, John W. Scott Health Sciences Library, University of Alberta 30 June 2004 The following explanatory information was provided in Table 1 of this article:"‡exp = explode, a search term that automatically includes closely related indexing terms."Based on my understanding of Ovid and PubMed search mechanisms as well as the information provided in the Ovid On-Screen Help, the above definition of the Explode operation could be misleading. A better definition would be one of the following:"Exp = explode, an operation that causes any conceptually narrower term(s) to be included automatically when a subject heading is searched." or alternately,"Exp = explode, an operation that causes subject headings that appear below the ‘parent’ term in the MeSH hierarchy to be included automatically when the parent term is searched." In the MeSH vocabulary, 'related' terms differ from 'narrower' terms. Narrower terms are specific types of whatever concept is represented by the original term. For example, the narrower terms for the MeSH term pain are: back pain (which in turn is the parent for the narrower term 'low back pain'), facial pain, headache, metatarsalgia, neck pain, neuralgia (parent for the narrower term 'sciatica') and pain, intractable. 'Exploding' the subject heading 'pain' will cause all the subject headings mentioned above to be included in the search.Related terms are subject headings which are not hierarchically related (i.e. broader or narrower) to the original subject heading but which still have a meaningful relationship to the original term. 'Related' (See Also) terms are NOT included when the subject heading 'pain' is exploded. If a searcher wishes to include any related terms, they should be searched separately.'Related' (See Also) terms for the MeSH term pain are: analgesia; analgesics; hyperalgesia; pain insensitivity, congenital and palliative care. In PubMed, the MeSH database can be used to identify narrower and related (See Also) terms. In Ovid MEDLINE, the MeSH Tree can be used to identify narrower terms and the subject Scope Note can be used to identify Related terms. Competing interests No competing interests.