The detail included explicitly in the music notation varies between genres and historical periods. In general, art music notation from the 17th through 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, unadorned melody. However, performers were expected to 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. The performer was expected to 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.
