Why is it called a raked stage?
Likewise, why is it called downstage?
So, Stage Right and Stage Left make sense. But why Upstage and Downstage? The terminology comes from the days in which the audience seats were on a flat floor and the stage was tilted (razed) toward the audience, so that everyone on the audience floor could see the performance.
In this manner, when was the raked stage invented?
Raked stages have been around since the 16th century, but have lost popularity in the past century, in favor of raking the audience seating area instead—think of the “stadium seating” you see at the movie theatre. To quote the New York Times, today “almost all American stages have flat floors.”
AEA says that any rake up to 3/4" per foot is acceptable. Over that you would have to add a rider to all the contracts and pay hazard pay. For ADA ramps (not exactly comparable to raked stages, but an acceptable analogy) "The maximum slope of a ramp in new construction shall be 1:12."