Out With The Old, In With The New

Here's a question for the masses, what year would you say "Old School Rap" ended and "New School Rap" began? Some additional questions could be... What do you define as Old and New School Rap? To you, what artists are considered Old School? New School? I'm sure the year(s) people suggest will be debatable, but that's the primary question I want to know the answer to. What year do you say?