In the '40s, hot jazz and cool pop crooners ceded the spotlight to a new breed of artists. Frank Sinatra took over the world and brought a jazz lover's musical moves to pop singing, changing the medium forever. Meanwhile, big-band swing kings were beginning to lose ground to outside-the-box bebop innovators like Dizzy Gillespie, opening up a whole atlas full of new routes for jazz to travel. The close, sweet harmonies of vocal groups like The Mills Brothers and The Andrews Sisters saturated the airwaves, especially during the WWII era, and when they mixed in shadings of jazz and jump blues, they contributed even further to the musical melting pot of the ‘40s, when genre boundaries became more porous than ever before. Along the way, country singers like Hank Williams found fans in pop audiences too, bringing the mainstream even more avenues of musical expression.