A musical number is a composition of music, usually written for performance by singers or instrumentalists. The term 'musical number' can refer to any of four different solutions: TUNE (4 letters), Opus (4 letters), SONG (4 letters), and PIECE (5 letters). A TUNE is a short, simple melody, often with repeating sections. An opus is a larger musical work, often composed of multiple movements. A SONG is a musical composition for voice and instruments, typically with words. A PIECE is a composition for one or more performers.