Jazz music is the iconic music of the 20th century, the direct ancestor of modern music. The original Southern 'trad' (traditional) jazz, based loosely on an earlier form called Ragtime, originated in the deep South, as Dixieland jazz. It moved north to Chicago, becoming the early form of modern jazz. During the Prohibition years, and with the assistance of Cole Porter, one of modern music's supergiants, it became mainstream music during the 1920s and 30s. It was originally black music, but was soon taken up by white musicians as well. The music continued to evolve with the Big Bands, particularly Count Basie and Duke Ellington and in the 1930s and 1940s developed into 'bebop' and 'swing'. This music went around the world, particularly taking hold in Europe. In the 1950s it began to evolve into an extremely advanced freeform type of music, 'fusion' jazz'. (The original rock and roll was strongly based on jazz guitar chords) In the 1960s it hit a high point, producing exceptional and innovative new musical concepts. Since then, jazz has been 'musician's music', a higher class of musical expression.