A quartet part is a role in a musical ensemble of four players. The two most common quartet parts are the VIOLA and ALTO, both of which have five and four letters respectively. The VIOLA is a string instrument that is slightly larger than a violin and has a deeper sound. It is usually played with a bow and is used to provide harmony and countermelody. The ALTO is a type of voice that is lower than a soprano and higher than a tenor. It is often used to provide a harmonic foundation for the other voices in a quartet.