One big change has been the Dempsey rule, mid-Twenties, that a fighter had to go to a neutral corner after knocking his opponent down. That had a HUGE impact, and is a clear divide between old and new styles---Dempsey was as famous and beloved in his era as Babe Ruth, and he used to just stand over a guy and beat snot out of him as soon as an opponent struggled to his feet. The mandatory eight count and similar rules changed the sport a whole lot.
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