Performance is the physical expression of music, which occurs when a song is sung or when a piano piece, electric guitar melody, symphony, drum beat or other musical part is played by musicians. In classical music, a musical work is written in music notation by a composer and then it is performed once the composer is satisfied with its structure and instrumentation. However, as it gets performed, the interpretation of a song or piece can evolve and change. In classical music, instrumental performers, singers or conductors may gradually make changes to the phrasing or tempo of a piece. In popular and traditional music, the performers have a lot more freedom to make changes to the form of a song or piece. As such, in popular and traditional music styles, even when a band plays a cover song, they can make changes to it such as adding a guitar solo to or inserting an introduction.