Onstage magazine has a short article that is very helpful in understanding the effect that speaker placement has on live sound. Monitor placement is particularly important for acoustic bands that have a large number of sensitive condenser microphones on stage. To achieve the best-sounding results and avoid ear-splitting feedback, you should understand the most effective monitor placement for the type of microphone you are using:

Purists will note that supercardioid mics like the venerable Shure Beta 58A have a very narrow front-pickup pattern, but they achieve this by sacrificing some of the rejection pattern directly behind the mic. Supercardioid mics are designed to work well with either a pair of floor wedges split 15 degrees off the center rear of the mic, or a single floor wedge slightly offset from the rear of the mic. If you point the back of a supercardioid mic directly at a monitor speaker, it will feed back more easily than if you offset it by 15 degrees or so.

To read the full article:
Location is Everything Courtesy of Onstage Magazine

Previous posts:
Know Your Mic, Part III
Know your mic, part II
Know your mic, part I
Plugging in