The only definition of sheet music that can be applied with certainty is to define it purely by physical format. Only the physical format remains constant.
Musical taste is, like art and fashion, subject to extreme changes and subtle nuances.
For more information about the physical description of sheet music, please consult the Music Library Association Sheet Music web site. The Rare Book, Manuscript, and Special Collections Library at Duke University holds an important, representative, and comprehensive collection of 19th and early 20th century American sheet music. The Historic American Sheet Music Project provides access to digital images of 3,042 pieces from the collection, published in America between 1850 and 1920. Drawn from the collections in the Rare Book, Manuscript, and Special Collections Library, this selection presents a significant perspective on American history and culture. The collection is particularly strong in antebellum Southern music, Confederate imprints, and Civil war songs.