The Pitts Special (company designations S1 and S2) is a series of light aerobatic biplane designed by Curtis Pitts. It has accumulated many competition wins since its first flight in 1944. The Pitts biplanes dominated world aerobatic competition in the 1960s and 1970s and, even today, remains a potent competition aircraft in the lower categories.
All single-seat (S-1) and two-seat (S-2) Pitts Specials are variations on the basic design from 1944. The design's popularity grew significantly following Bob Herendeen's participation on the USA Aerobatic Team in a Pitts Special in the World Aerobatic Competition in Moscow, Russia in 1966. In 1972, the US National Aerobatic Team won the World Championships flying only Pitts biplanes.
Source: Wikipedia, "Pitts Special", available under the CC-BY-SA License.