There are 83 topical qualifiers used for indexing and cataloging in conjunction with descriptors. Qualifiers afford a convenient means of grouping together those citations which are concerned with a particular aspect of a subject. Not every qualifier is suitable for use with every subject heading. In the following list of qualifers, the Tree categories in parentheses are a general guide to the descriptors permitted with the qualifier. Generally a qualifier may be used with the descriptors in a given subcategory but there may be exceptions and there may be a few descriptors in subcategories not listed which are permitted with the descriptor.(http://www.nlm.nih.gov/mesh/topsubscope2007.html)