Contemporary dance consists of various styles of dance, which are often classified by the era in which they were created or popularized. Contemporary dance is sometimes considered to include post–modern dance and sometimes is considered to be synonymous with Modern dance. Contemporary dance emerged in the early 20th century after the development of modernism in the arts. The first decade of the 20th century was marked by a return to traditionalism in the arts, which was followed by a period of experimentation. The most significant pioneer of contemporary dance was Isadora Duncan, who rejected the strict technique of ballet in favor of a freeform style of expressive dancing. Other early pioneers include Ruth St. Denis, Loie Fuller, and Marie Rambert. Contemporary dance began to develop its own identity in the 1920s and 1930s, with the works of Mary Wigman, Doris Humphrey, and Martha Graham. Graham‘s work was particularly influential, and her technique became the standard for modern dancers. The mid–20th century saw the development of post–modern dance, which was a reaction against the strictures of modernism. Post–modern dance is often characterized by a rejection of traditional forms and an embrace of improvisation and experimentation. Notable contemporary dancers include Pina Bausch, Merce Cunningham, Twyla Tharp, and Bill T. Jones.