Music is often preserved in memory and performance only, handed down orally, or aurally ("by ear"). When the composer of music is no longer known, this music is often classified as "traditional". Different musical traditions have different 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 music notation, and the study of how to read notation involves music theory, harmony, the study of performance practice, and in some cases an understanding of historical performance 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 standard musical notation is the lead sheet, which notates the melody, chords, lyrics (if it is a vocal piece), and structure of the music. Nonetheless, scores and parts are also used in popular music and jazz, particularly in large ensembles such as jazz "big bands."
In popular music, guitarists and electric bass players often read music notated in tablature, which indicates the location of the notes to be played on the instrument using a diagram of the guitar or bass fingerboard. Tablature was also used in the Baroque era to notate music for the lute, a stringed, fretted instrument.
Generally music which is to be performed is produced as sheet music. To perform music from notation requires an understanding of both the musical style and the performance practice that is associated with a piece of music or genre. The detail included explicitly in the music notation varies between genres and historical periods. In general, art music notation from the 17th through to the 19th century required performers to have a great deal of contextual knowledge about performing styles.
For example, in the 17th and 18th century, music notated for solo performers typically indicated a simple, unornamented melody. However, it was expected that performers would know how to add stylistically-appropriate ornaments such as trills and turns.
In the 19th century, art music for solo performers may give a general instruction such as to perform the music expressively, without describing in detail how the performer should do this. It was expected that the performer would know how to use tempo changes, accentuation, and pauses (among other devices) to obtain this "expressive" performance style.
In the 20th century, art music notation often became more explicit, and used a range of markings and annotations to indicate to performers how they should play or sing the piece. In popular music and jazz, music notation almost always indicates only the basic framework of the melody, harmony, or performance approach; musicians and singers are expected to know the performance conventions and styles associated with specific genres and pieces.
For example, the "lead sheet" for a jazz tune may only indicate the melody and the chord changes. The performers in the jazz ensemble are expected to know how to "flesh out" this basic structure by adding ornaments, improvised music, and chordal accompaniment.
