National Library of Medicine classification 美国国家医学图书馆分类法
The National Library of Medicine (NLM) classification system is a library indexing system covering the fields of medicine and preclinical basic sciences. The NLM classification is patterned after the Library of Congress (LC) Classification system: alphabetical letters denote broad subject categories which are subdivided by numbers. For example, QW 279 would indicate a book on an aspect of microbiology or immunology.