Barbershop music encompass both men's and women's singing in the barbershop style – in quartets and choruses. Sweet Adelines International and Harmony, Inc. are two women's barbershop singing organizations that operate globally and in North America, respectively. Other women's organizations include the Ladies Association of Barbershop Singers (LABBS) in the United Kingdom, the Spanish Association of Barbershop Singers (SABS) in Spain, and the Irish Association of Barbershop Singers (IABS) in Ireland. A barbershop quartet is an ensemble of four people who sing a cappella in the exacting barbershop music genre. In North America most male barbershop quartet singers belong to the Barbershop
Barbershop music of the "revivalist" style are related to the desire to produce these ringing chords. Performance is a cappella to prevent the distracting introduction of equal-tempered intonation, and because listening to anything but the other three voices interferes with a performer's ability to tune with the precision required. Barbershop arrangements stress chords and chord progressions that favor "ringing", at the expense of suspended and diminished chords and other harmonic vocabulary of the ragtime and jazz forms. The dominant seventh-type chord is so important to barbershop harmony that it is called the "barbershop seventh". BHS arrangers believe that a song should contain