All of time can be divided pretty easily this way:- Everything before "Run DMC" (Run DMC [1984])
- Everything before "It Takes A Nation of Millions To Hold Us Back" (Public Enemy [1988])It gets harder after that, but maybe you'd have to label parallel movements with stuff like:
+ Everything before "Straight Outta Compton" (NWA [1988]) and "Amerikkka's Most Wanted" (Ice Cube [1990]) for the West Coast Gangsta Movement
+ Everything before "3 Feet High And Rising" (De La Soul [1989]) and "Low End Theory" (A Tribe Called Quest [1991]) for the Jazz/Bohemian MovementOthers seem to think we're about to enter the Everything before "93 'Til Infinity" (Souls of Mischief [1993]) era for the Ascendency of Hieroglyphics, but I'll reserve judgement on that.
There are also several dark moments: MC Hammer ushered in the HipPop Era and Vanilla "No I really used to be rich; May I take your order?" Ice ushered in something unspeakable (Snow, if nothing else).
Hmmmmm, here's a way to pass the time: what are the key moments/movements and what albums/artists best represent them? How have they influenced the direction of rap? Making no value judgements, I'd have to say that the four most influential hiphop moments/movements are the ones I listed above and for obvious reasons.