It can follow the standard voice-leading rules of classical harmony, as in string quartets or Bach chorales, or proceed in parallel motion with the melody in thirds or sixths.
Artists like The Beatles and Simon & Garfunkel used close harmony, echoing their chosen role-models, The Everly Brothers.
The bass line tends to be more rhythmic and covers the root notes of the harmonic progression, providing more "support" and independence than in classical vocal music, since Barbershop is usually sung a cappella.
Originally called "race music" by white mainstream radio and its target market, it was the precursor to rock and roll and rhythm and blues of the 1950s, 1960s and 1970s, influencing many English and American artists of that era.
They often used the more traditional TTBB or SSAA 4-part structure, but with heavy use of solos and call-and-response, which is rooted in the African American church.
[6] A well-known example of consistent instrumental close harmony is Glenn Miller's "Moonlight Serenade" which uses the full range of single-reed wind instruments (soprano clarinet, alto, tenor, and baritone saxophones) to make a distinctive sound by harmonizing the different sections all within a single octave.