The 1940s jazz style was characterized by two distinct styles: BEBOP and BOP. BEBOP, also known as 'rebop' or 'modern jazz', was developed in the mid-1940s and is characterized by fast tempos, complex harmonic structures, and improvisation. BOP, also known as 'hard BOP', was developed in the late 1940s and is characterized by a more traditional approach to jazz, with slower tempos and simpler harmonic structures. Both styles of jazz were popular during the 1940s and remain popular today.