What is "four-part harmony"

n. harmony in which each chord has four notes that create four melodic lines

The term "four-part harmony" refers to music written for four voices, or four musical instruments, or a keyboard instrument, or some other medium, where the various parts give a different note of each chord of the music. Typically, the first of the four parts will sing (or play) the melody, with the other three parts providing the supporting harmonies. It is unusual for any of the four parts to share the same pitch, although it happens at times.

The four main voices are typically labelled as: soprano (or treble), alto ( contralto or countertenor), tenor, and bass. Because most singers have a relatively limited range, the upper notes of the soprano or tenor part cannot be sung by a bass singer. Conversely, the lower notes of the bass part typically cannot be reached by a soprano voice, with some notes so low that alto and tenor voices cannot reach them either.

Groups of just four people, singing as quartets, can perform in four-part harmony.