The primary sources of a musical work can include the following:
Autograph / Holograph (the composer's own manuscript)
Copies (handwritten by a relative, student, colleague, or professional copyist; or, in the case of medieval works, by monastic scribes)
First edition (typically published in consultation with the composer)
Early editions (printed during the composer's lifetime)
Early editions (edited by a relative, student, or other person close to the composer, after the composer's death)
Scholarly editions (edited by a scholar or performer known for their knowledge / interpretation of the composer)
Collected editions (the composer's complete works, published in a scholarly edition that shows the variant readings (differences) in the above sources)
Once you've identified the surviving primary sources of a work, and the libraries that own them, how do you get to see them? Some sources, of course, are too rare or fragile to be loaned via interlibrary loan. You can travel to the holding libraries. Or, for many early sources, various kinds of reproductions are available:
Printed facsimiles of manuscripts
Search these in the Library's catalog or in WorldCat, by composer and/or title, or with the following subject heading:
For facsimiles of medieval collections, try subject headings like these:
These published facsimiles are generally available via interlibrary loan, if BYU doesn't have them.
Reprints of early editions
Some publishers have produced photographic reprints of early printed editions. Search them in the Library's catalog or WorldCat by composer and/or title. A note in the record will indicate that it's a reprint, and give the date, etc. of the original edition.
These reprints are generally available via interlibrary loan, if BYU doesn't have them.
Digital images on the web
Many libraries have begun digitizing their primary source materials. Once you know which library owns a specific manuscript or edition, check the library's website to see if a digital image is available.
The following sites give access to major research libraries:
The European Library (gateway site to European national libraries)
Beethoven-Haus Bonn (manuscripts, etc. of Beethoven)
Don't forget our own Rare Books, Baptist, and microtext collections. Whenever you do a search in the Library's catalog for a specific work, composer, or subject, any relevant primary sources we have in our Special Collections will be retrieved along with our other materials.