Quartets - "Four Square"
Quartet "Four Suare" (John Denton, Dave King, Bruce Warren, Charles Oram) in competition.
Many see quartet singing (and especially quartet contests) as the pinnacle of the barbershop experience. Certainly, more experience is required - there is "nowhere to hide" in a quartet, as there are no longer 20 other singers also singing your part.
City of Sails generally has anything from one to five quartets going at any one time.