One of the unique qualities of barbershop music is the barbershop seventh chord. This is basically a major-minor seventh chord, or a major chord with a minor seventh above the root. The unique aspect of this chord in barbershop music is that the seventh is sung as a blue note, or slightly lower in pitch than normal.
This chord is typically performed as the dominant seventh chord in the key of the composition. The dominant seventh chord typically leads back to the tonic chord in the chord progression, however, this sound can also be heard in other chords, particularly the subdominant chord in the key of the composition.
It is important to understand that chordophones are tuned using equal temperament, where each of the twelve semitones in the octave are tuned equally. Since the barbershop seventh chord uses a slightly lowered pitch, a chordophone is typically not able to effectively perform this chord. Instruments (including voice) that are able to perform with just intonation can accurately perform a barbershop seventh chord.