The breakdown of form and rhythmic structure has been seen by some critics to coincide with jazz musicians' exposure to and use of elements from non-Western music, especially African, Arabic, and Indian. The atonality of free jazz is often credited by historians and jazz performers to a return to non-tonal music of the nineteenth century, including field hollers, street cries, and jubilees (part of the "return to the roots" element of free jazz). This suggests that perhaps the movement away from tonality was not a conscious effort to devise a formal atonal system, but rather a reflection of the concepts surrounding free jazz. Jazz became "free" by removing dependence on chord progressions and instead using polytempic and polyrhythmic structures.