From Sweet Adelines International:
Barbershop harmony is unaccompanied, four-part a cappella harmony.
Although barbershop-style music is usually built on simple melodies and is relatively easy to sing, the a cappella style and the ear training necessary for independent part singing make it one of the most challenging and rewarding accomplishments of a vocal ensemble. When the music is sung accurately and with good breath support and vocal techniques, barbershop harmony produces overtone vibrations that create a resonant ring unique to this form of music.
Women of all skill levels join us, and their abilities are as varied as they are. It's the coming together of these unique voices, talents and experiences that creates the space for an exciting ensemble.
The voice parts in barbershop harmony for women have different names and functions than they do in other SATB or SSAA vocal styles. The LEAD voice generally sings the melody and is below the TENOR harmony; the TENOR part sings the highest note in the chord; the BARITONE part fills in the all-important missing note in a chord that may be above or below the meldoy; the BASS part supplies the harmonic foundation (root or fifth) of the chord. Similar to choral music, minimal vibrato should be apparent in barbershop singing. Wide and obvious vibratos tend to hamper the "lock and ring" that we look for in our chords.