A part (or voice) is a strand or melody of music played by an individual instrument or voice (or group of identical instruments or voices) within a larger work. Parts may be referred to as an outer part (the top and bottom parts) or an inner part (those in between). Part-writing (or voice leading) is the composition of parts in consideration of harmony and counterpoint. In the context of polyphonic composition the term voice may be used instead of part to denote a single melodic line or textural layer. The term is generic, and is not meant to imply that the line should necessarily be vocal in character, instead referring to instrumentation or simply to register.

Vocal music is a type of music performed by one or more singers, with or without instrumental accompaniment (a cappella), in which singing provides the main focus of the piece. Music which employs singing but does not feature it prominently is generally considered instrumental music (e.g. the wordless women's choir in the final movement of Holst's The Planets) as is music without singing. Music without any non-vocal instrumental accompaniment is referred to as a cappella.