The style of Jazz regarded by the average player as today's norm, as opposed to fusion, rock, avant-garde, etc. ; sometimes the term implies a somewhat conservative, relatively diatonic vocabulary exemplified by Oscar Peterson. Mainstream Jazz is in a highly evolved state, having incorporated virtually the entire harmonic language of 20th century tonal music. In timbre, phrasing, form and rhythmic feel mainstream Jazz still rests on a basis of Bebop, which is why 'modern' Jazz is considered to have started with Bebop in the early 40s.