The card catalog remains an important tool for researchers and Library of Congress reference librarians attempting to verify books and periodicals held by the Library. Not all card catalog records were successfully migrated to the online catalog; consequently, sometimes the card catalog includes the only record that the Library of Congress held, or holds, a particular title. Given this, it is still best practice to check the card catalog for books published before 1981 not found in the online catalog if there is a reason to believe the Library holds a copy.

In addition, many cards contain extra information not found in the online catalog, such as subtitles, contents notes, and series statements. The image below is for a record in the online catalog for a book from 1858.

Enough information is available to request the book, but more information about this title might be helpful. Below is an image of the card for this title which contains additional details about this item, including the book's full title, which gives a better indication of its subject.

A card catalog may also include information such as title changes for periodicals and the location of a book in a special collections division such as the Rare Book and Special Collections Reading Room. These additional details may prove useful to researchers trying to trace a work's publication history or its availability and location in the Library's permanent collections.