Jazz is a musical genre born in the early twentieth century as an evolution of musical forms already used by African American slaves. Initially it had the form of "work songs" (work songs) in plantations and during the construction of railways and roads in the United States and served to pace and coordinate movements (the rhythm was binary). Early musicians played music by ear, and the pioneering orchestras in New Orleans were called ragtime bands. A notable contribution to a first instrumental evolution of jazz was given by the Italian emigrants of New Orleans who precisely added other musical instruments from the Italian tradition of village bands: in fact, among the best jazz musicians there were African Americans who were later joined by Italian Americans, such as Nick La Rocca who formed the Original Dixieland Jazz Band, i.e. the band that spread jazz in the U.S.A. producing many records. Jazz will arrive in Chicago with Louis Armstrong and then in Europe where it will have a huge success. Over the years it will change and will also become commercial music with swing until it resumes the traditions of African-American culture of the first jazz bands with bebop. In jazz there are two main forms: the blues, in 12 bars (3 musical phrases), and the song, in 32 bars. Initially the essence of improvisation was in the melodic line, this is due to the fact that the prototypical (original) jazz medium is the brass ensemble, in which, given that each player can only produce one note at a time, solos are necessarily melodic. Accompanying harmonic instruments (piano, guitar, double bass) were introduced later. From the earliest times, jazz has incorporated into its language the genres of popular music, ragtime, blues, pop music and finally cultured music, especially American. In more recent times, jazz has also mixed with all modern musical genres, including non-US ones, such as samba, Caribbean music and rock. Jazz has transformed over the course of the twentieth century, evolving into a great variety of styles and subgenres: from the dixieland of New Orleans in the early years, to the swing, of the big bands in the thirties and forties, from the bebop of the second half of the forties, to the cool jazz and hard bop of the fifties, from the free jazz of the sixties to the fusion of the seventies, up to the contaminations with funk and hip hop of the following decades. The use of these labels has not been very welcome by many (jazz) musicians who prefer to define their music simply as jazz. After the seventies, jazz entered the so-called cultured music, thus entering the courses held in music schools and conservatories.