What is the rear center channel for and what type of speaker should I use?

The rear center channel is a recent extension of both Dolby Digital and DTS 5.1 processing, called EX and ES respectively. It is based on the same principle as the front center channel which is to "localize" sounds in the center of the soundstage or give them a distinct area they are appear to come from, filling in the sound gap between the left and right rear channels. Therefore, the same type of speaker used for the front center may be used for the rear center. Because of the need to "localize" the sound to the center, a monopole center channel speaker is usually the best choice. Some electronics offer the option of using TWO rear center speakers. What will sound best is dependent on the size of the room and what type of surround speakers you choose. WDST™ surrounds generally do not require the addition of a rear center, but adding one enhances the localization of events in the rear sound field.