A short history lesson
(Excerpts from the websites of the Cheadle U3A Barbershop Singing Group, UK, and from the Barbershop Harmony Society)

“Barbershop music, with its close, unaccompanied, four part harmonies, is a unique American folk art. It was taking form between 1860 and 1920. In the early years, singers would improvise the harmonies but when the printing press was adapted to produce musical notation, there was further advancement of the barbershop idiom. At the turn of the century, amateur singers, mainly men, were often heard singing improvised barbershop harmonies at parties and picnics. Minstrel shows often featured barbershop quartets. The influence of barbershop music on other popular forms of singing is profound. "Old Blue Eyes" himself, the great Frank Sinatra, sang in a barbershop quartet before he became famous as a singer with the Tommy Dorsey band. The vocal quartets of bands such as Dorsey's and Glenn Miller's were also greatly influenced by the barbershop style. In the latter half of the 20th century, the close harmony style has been the hallmark of "pop"  and jazz groups such as the Beach Boys, The Four Seasons and the Four Freshmen. True barbershop lives on however, kept alive by dedicated barbershop choruses, groups and organisations that include Sweet Adelines International and its individual members”.

The Barbershop Harmony Society is devoted to promoting, preserving, and enjoying barbershop harmony. But what makes a particular song or arrangement "barbershop-able"? What's the difference between barbershop and doo-wop, jazz, madrigal, and other a cappella music? Technically speaking, barbershop harmony is a style of unaccompanied singing with three voices harmonizing to the melody. The lead usually sings the melody, with the tenor harmonizing above the lead. The bass sings the lowest harmonizing notes and the baritone provides in-between notes, either above or below the lead to make chords (specifically, dominant-type or "barbershop" sevenths) that give barbershop its distinctive, "full" sound. Probably the most distinctive facet of barbershop harmony is the phenomenon known as expanded sound. It is created when the harmonics in the individually sung tones reinforce each other to produce audible overtones or undertones. Barbershoppers call this "ringing a chord." Singing in a quartet or chorus and creating that "fifth voice" is one of the most thrilling musical sensations you'll ever experience, leading to goosebumps the size of golf balls.