Bc and ad, plz explain.?

How bloke y years in bc before ad appeared? ad is after christ and bc is before but what yr does bc go up to for it to become ad?
Answer
Bc goes up to 0 (that being the point at which christ came), anno domini (the year of our lord) goes from then onward. so, 12 bc was 12 years before christ for example, and 12 ad was 12 years after his birth.