A lot of the information above is accurate but everyone has missed the reason for why a stage is raked. In the 16th and 17th century, the SEATING area in most theatres was flat and level. So theaters raked the stage so that everyone would have a good view of what was going on onstage. Without a raked stage, only the first row or two would see the entire view. And it gets progressively worse. With a flat stage, by row H or I, the audience would only see the cast from the waist up. So they raked the stage. Also remember back then, in operas, the chorus did little stage movement - they were assembled onstage, ready to belt out as the curtain rose on each scene, so they could REALLY rake the stage. Sometimes, they hads up to a rise/run of 4:16 (a four foot rise every 16 feet. That would be like that twisty street in San Francisco). The chorus would literaly have to climb offstage ladders to get onto the set, while the leads acted "in one" at the level front of the stage.
http://www.broadwayworld.com/board/readmessage.cfm?thread=972787#3631451
http://www.broadwayworld.com/board/readmessage.cfm?thread=963561#3533883
http://www.broadwayworld.com/board/readmessage.cfm?thread=955158#3440952
http://www.broadwayworld.com/board/readmessage.cfm?thread=954269#3427915
http://www.broadwayworld.com/board/readmessage.cfm?thread=955012#3441622
http://www.broadwayworld.com/board/readmessage.cfm?thread=954344#3428699