Sometimes the challenge in locating a piece of music lies in verifying the facts. Some pieces have opus numbers, other numbering (e.g., Quartet No. 2), key designations (e.g., E-flat major), or even nicknames (e.g., the American Quartet). Knowing this information can help you in locating this piece in the Library. The two major types of tools to use for verifying this type of information are 1) the New Grove Dicitionary of Music and Musicians works lists and, 2) the composer's thematic catalog.