The center speaker is the single most important loudspeaker in a 5.1, 6.1 or 7.1 surround sound system, as it is responsible for much of the dialogue of film soundtracks. Center speakers are located near and normally somewhere below the actual video screen. In the case of a large-scale home theater system, you might find the center speaker installed behind a perforated video screen, with the sound emitting through small holes in the screen. It is common to find powerful horn speakers for this type of application.
For home theater use, it is important to use timbre-matched speakers across your front three speakers. If you are using MartinLogan main speakers for your left and right front channels, it wouldn't be in your best sonic interest to use, say, a Bose center speaker if you are looking for a uniform sound across your front three speakers.