A lead sheet or fake sheet is a form of musical notation that specifies the essential elements of a popular song: the melody, lyrics and harmony. The melody is written in modern Western music notation, the lyric is written as text below the staff and the harmony is specified with chord symbols above the staff.
The lead sheet does not describe the chord voicings, voice leading, bass line or other aspects of the accompaniment. These are specified later by an arranger or improvised by the performers, and are considered aspects of the arrangement or performance of a song, rather than a part of the song itself.
A lead sheet may also specify an instrumental part or theme, if this is considered essential to the song's identity. For example, the opening guitar riff from Deep Purple's "Smoke on the Water" is a part of the song; any performance of the song should include the guitar riff, and any imitation of that guitar riff is an imitation of the song. Thus the riff belongs on the lead sheet.
A collected volume of lead sheets may be known as a fake book, due to the improvisational nature of its use: when presented with a lead sheet, proficient musicians may be able to "fake it" by performing the song adequately without a full score. This is in contrast to a full score, in which every note to be played in a piece is written out. Since fake books and lead sheets only give a rough outline of the melody and harmony, the performer or arranger is expected to improvise significantly.