Ax, as Jay pointed out, that concept of being spread about as widely as the mains applies to back surround speakers in a 7.1 setup, spreading them about as much as they're behind the listeners, in analogous fashion to the spread of the mains.

Since the 5.1 surrounds are side surrounds, they can be spread almost as far as they would be if they were actually mounted on the side walls rather than the back wall.

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Enjoy the music, not the equipment.