This is one of the most common ways to set up a room for listening, simply because it often allows for enough space to form a good listening triangle while keeping both your head and the loudspeakers away from most wall boundaries. A general set up will look like the diagram below.
In either case, once you have your speakers placed, the goal is to position your listening chair in order to form an equilateral triangle with the loudspeakers.
All of the scenarios offered are simply starting points and assume an ideal situation where furniture placement does not play a factor in your system set up. If you find that you can not place the speakers well in to the room, just be sure that you follow the ratios of rear and side wall placement, where distance A is proportional to distance B. In some situations this may result in a much wider distance between the speakers. In this case you could make the listening triangle larger or if that is not possible, then you would need to toe in the loudspeakers more dramatically to achieve a strong center image. Of course if the distance between the speakers becomes too extreme, you could always reverse distance A and distance B in order to maintain the same ratio of side wall and rear wall placement.