Crosby, Stills & Nash (CSN) is a folk rock supergroup made up of David Crosby, Stephen Stills and Graham Nash, also known as Crosby, Stills, Nash & Young (CSNY) when joined by occasional fourth member Neil Young. They are noted for their intricate vocal harmonies, often tumultuous interpersonal relationships, political activism, and lasting influence onAmerican music and culture. All four members of CSNY have been inducted into the Rock and Roll Hall of Fame twice,though Young's inductions were for work not involving the group.

Graham Nash had been introduced to Crosby when the Byrds had toured the United Kingdom in 1966, and when the Hollies ventured to California in 1968, Nash resumed his acquaintance with Crosby.At a party in July 1968 at Joni Mitchell's house, Nash asked Stills and Crosby to repeat their performance of a new song by Stills, “You Don't Have To Cry”, with Nash improvising a third part harmony.The vocals jelled, and the three realized that they had a unique vocal chemistry.

http://en.wikipedia.org/wiki/Crosby,_Stills,_Nash_%26_Young