Vocal harmonies

Vocal harmony is a style of vocal music in which a consonant note or notes are simultaneously sung as a main melody in a predominantly homophonic texture. Vocal harmonies are used in many subgenres of European art music, including Classical choral music and opera and in the popular styles from many Western cultures ranging from folk songs and musical theater pieces to rock ballads. In the simplest style of vocal harmony, the main vocal melody is supported by a single backup vocal line, either at a pitch which is above or below the main vocal line, often in thirds or sixths which fit in with the chord progression used in the song. In more complex vocal harmony arrangements, different backup singers may sing two or even three other notes at the same time as each of the main melody notes, mostly with consonant, pleasing-sounding thirds, sixths, and fifths (although dissonant notes may be used as short passing notes).

Vocal harmonies have been an important part of Western art music since the Renaissance-era introduction of Mass melodies harmonized in sweet thirds and sixths. With the rise of the Lutheran church's chorale hymn singing style, congregations sang hymns arranged with four or five-part vocal harmony. In the Romantic era of music during the 1800s, vocal harmonization became more complex, and arrangers began including more dissonant harmonies. Operas and choral music from the Romantic era used tense-sounding vocal harmonies with augmented and diminished intervals as an important tool for underscoring the drama of the music. With contemporary music from the 1900s and 2000s, composers made increasingly difficult demands on choirs which were singing in vocal harmony, such as instructions to sing microtonal notes or make percussive sounds.

...
Wikipedia