Due to copyright restrictions, much music is unavailable on the Internet unless you are looking to purchase copies. Most sheet music is copyrighted for 95 years after the date of publication; as of January 1, 2021, music published before or during 1925 is presumed to be in the public domain. Sheet music sites below deal with historical, public-domain music.

• IMSLP / Petrucci Music Library (Large number of user-uploaded public domain scores)
• Sheet Music Consortium (Includes Johns Hopkins University, UCLA, Indiana University, Duke University, and others)
• Library of Congress collections
• Choral Public Domain Library 
• Julliard Manuscript Collection

ASpace has a large number of sheet music collections. Below are some featured collections:

• Lester S. Levy Sheet Music Collection
  • Over 29,000 pieces of American popular music from Special Collections at the Milton S. Eisenhower Library of The Johns Hopkins University. Especially strong in music spawned by military conflicts from the War of 1812 through World War I as well as minstrel music.
• Sam Carner Sheet Music Collection
  • The Sam Carner sheet music collection, 1830-1978, includes 19th and 20th century popular sheet music, musical magazines, and song compilations. The collection is particularly strong in African American contributors (including several first editions of songs by W.C. Handy), songs published in Baltimore, minstrel songs, patriotic songs, and Yiddish songs. There are also a significant number of uncategorized popular songs.
• Patti Chapin Sheet Music Collection

  • Patti Chapin (later Patti Steincrohn) was a CBS radio singer and pianist from approximately 1934-1943. The collection, approximately 1900-1970, primarily contains annotated and unannotated performance (artist) copies of popular songs, but also includes correspondence, images, song catalogs, unverified original compositions, and various collected popular songs.

Musical Instrument codes are used in much music cataloging to identify the instruments used in a musical work. They consist of a four place code--the first two places are an alphabetical code for type of instrument or voice; the second two places show the number of performers. 

For example, ba is the code for horn, thus ba01 is the code for one horn and ba02 indicates a work with two horns. To specify a work which includes a soprano, a clarinet and a piano set the keyword to musical instrument code, and enter va01 ka01 wc01.

The codes are classed by families: