found: Work cat.: Fitch, E.M. Thru the years with the Harmonizers, c1984:p. 1 (barbershop singing)
found: Web. 3(Barbershop : barbershop singing)
found: LC data base, 5/1/85(barbershopping)
found: New Grove dict. of mus. WWW site, June 3, 2010:Barbershop (style of unaccompanied singing that originated in the USA in the late 19th century. It is characterized by four-part harmony using chords that contain tritones...From the 1940s on, barbershop harmony was sung either by male or by female quartets and choruses, but in the later 20th century mixed-voice groups gained increasing popularity.)