The Supplementary Catalog contains Gaylord Music Library Special Collections materials not cataloged in the general Library Catalog. It brings together a collection of non-circulating music outside of the fully cataloged collection as a finding tool and aid to research.

At present, this catalog contains over 28,000 entries for material in four major categories: sheet music published in St. Louis and Missouri as well as music published on Missouri themes and topics, and elsewhere to a lesser extent; "popular" sheet music, including songs from musicals, motion pictures, and "standards"; music manuscripts, principally of musicians with ties to St. Louis and Washington University; and vocal and instrumental "art" music in the "green-box collection," described in collection-level records. It also contains records for individual pieces of music that are contained in selected made-up collections, published collections, and music periodicals cataloged in the regular Washington University Libraries Online Catalog.