The Library of Congress Online Catalog, the primary catalog for the books and periodicals in the General Collections, is the only catalog for such materials cataloged after 1980 (URL: catalog.loc.gov). It is available 24/7 to remote users through the Library's Website. The Library's Online Catalog also contains records for many materials cataloged before 1980, but to avoid the problem of trying to determine former subject headings and other difficulties in the online catalog, researchers on site may also want to use the 22,000-drawer Main Card Catalog (MCAT) housed just off the Main Reading Room. Cards were filed into the MCAT until 1980.

The MCAT remains the most accurate and complete place to identify most of the Library's book and periodical holdings published before 1980. The MCAT has consistent subject headings derived from the ninth edition of the Library of Congress Subject Headings (1980). Copies of this edition are available near the card catalog. This means, for example, that although you must search in the online catalog under both the current term “Women” and the former heading “Woman” to find all relevant records, in the Main Card Catalog all records with the old term “Woman” were recataloged so that they are filed together under the newer term “Women.” The MCAT also often contains notes, serial holdings, and other information that is not available online. For many items in the General Collections, the only record is in the card catalog.