Music is often conserved in memory and performance only, handed down orally, or aurally. Then the composer of music is no longer known this music is often classified as "traditional". Different musical traditions have dissimilar attitudes towards how and where to make changes to the original source material, from quite strict, to those which demand improvisation or modification to the music. In the Gambia, West Africa, the history of the country is passed orally through song.
When music is written down, it is generally notated so that there are instructions regarding what should be heard by listeners, and what the musician should do to perform the music. This is referred to as musical information, and the study of how to read notation involves music theory, harmony, the study of performance practice, and in some cases and understanding of historical presentation methods. Written notation varies with style and period of music. In Western Art music, the most common types of written notation are scores, which include all the music parts of an ensemble piece, and parts, which are the music notation for the individual performers or singers. In popular music, jazz, and blues, the regular musical notation is the lead sheet, which notates the melody, chords, lyrics, and structure of the music. Nonetheless, scores and parts are also used in popular music and jazz, mainly in large ensembles such as jazz "big bands."