Copyright is secured automatically when the work is created, and a work is "created" when it is fixed in a copy or phonorecord for the first time. "Copies" are material objects from which a work can be read or visually perceived either directly or with the aid of a machine or device, such as books, manuscripts, sheet music, film, videotape, or microfilm. "Phonorecords" are material objects embodying fixations of sounds (excluding, by statutory definition, motion picture soundtracks), such as cassette tapes, CDs, or LPs. Thus, for example, a song (the "work") can be fixed in sheet music ("copies") or in phonograph disks (" phonorecords"), or both.

If a work is prepared over a period of time, the part of the work that is fixed on a particular date constitutes the created work as of that date.

http://www.loc.gov/copyright/circs/circ1.html#hsc