Swing jazz began in the 1930s, dominated the 40s, and then was supplanted by "Modern Jazz", particularly Bop. Swing built on all the jazz that came before, but the crucial change was that from New Orleans through Kansas City, jazz emphasized the 1 and 3 beats (heard most clearly in "Kansas City Stomp" bands like Benny Moten's), and with swing, what MADE something "swinging" was the switch to an emphasis on the 2 and 4 beats. So "Swing and Before" could also be described as jazz from the beginning until Bop. There is no settled opinion about who was the key transitional figure as jazz moved from the "1-3" emphasis to the "2-4". The key transitional figure as jazz moved from swing to "Modern" is said to be Lester Young, whose solos were often modern even though he was embedded in a swing band.