"Library of Congress Subject Headings (LCSH) has long included headings that represent the genre or form of musical works in combination with the mediums of performance (e.g., Sonatas (Trombone and organ); Concertos (Bassoon, clarinet, English horn, flute)), as well as headings that represent the mediums of performance alone (e.g., Bass clarinet and piano music). The structure of the headings can sometimes be of marginal utility to researchers, who may be most interested in finding music for a particular combination of instruments and are less focused on the genre of the music.
Over the coming year, the concepts in the existing LC subject headings will be broken into their component parts. The forms and genres of musical works will be included in Library of Congress Genre/Form Terms for Library and Archival Materials (LCGFT) in 2014 or early 2015. The mediums of performance will be contained in the Library of Congress Medium of Performance Thesaurus for Music (LCMPT). When LCMPT and LCGFT terms for music are both implemented, the LC subject headings will cease to be assigned in new cataloging and will eventually be cancelled.
LCMPT is a stand-alone vocabulary that provides terminology to describe the instruments, voices, etc., used in the performance of musical works. (...) The initial 802 LCMPT terms are based chiefly on existing LC subject headings, but some additional terms that do not already appear in LCSH have also been included. Authorized terms and references in LCMPT generally consist of single words and phrases, but parenthetical qualifiers are occasionally employed to differentiate among homonyms. All terms and references are in the singular form and are lowercased unless they are proper nouns (e.g., flute; saxophone ensemble; but Irish harp)
The thesaurus has three broadest terms: ensemble, instrument, and performer. Each of the other terms is hierarchically subordinate to one or more of these terms and exhibits the class/class member relationship. Most of the authorized terms have Used For (UF) references for synonyms. Scope notes are also provided in many cases, and may describe the medium’s physical structure, the time period in which it was popular, and/or its geographic origin."