For the above and other reasons, musicologists have often found it necessary to produce comprehensive catalogues that incorporate the most up-to-date information available about the composers' works.
These catalogues sometimes also include unpublished sketches, incomplete drafts, even doubtful works and those known to have been spuriously attributed, as well as writings and other non-musical output.
Among the most famous examples of this are: In some cases, both the opus number and the newer catalogue designation are appended to a work.
[2] Such catalogues can be used for many purposes, including as guides to a specific composer's works, as an inventory of a library's holding or as an advertisement of a publisher's output.
In addition to the musical identification, a thematic catalogue may contain information such as dates of composition and first performance.
Thematic catalogues produced as scholarly guides to the works of a particular composer provide a shorthand means of identification for their music.