Surrounds vs. Surround Back

When using 4 surround speakers in a 7.1 layout, the "surround" speakers are the ones placed along your sides while the "surround-back" speakers are the ones that go behind you.

For a 5.1 layout, plug your 2 surround speakers in the connections marked "surround", irrespective of where you physically place them (would be preferable along your sides).

Well, I had my Mirage OMD-5's in the back in a 5.1 configuration for a long time. One day I decided to rearrange and put them on the sides instead. Huge difference, the surrounds sound way better on the sides than they do in the back. The sound is much more enveloping to me. Right now I have the best of both worlds though, 2 on the sides and 2 in the back and it sounds even better yet.

The diagrams represent the recommendations and are meant to be a guide. An ideal setup is not always possible. Do the best you can with what you have to work with. You can get reasonable results with them placed behind instead of to the sides. Within reason, the further apart, the better, probably. As sdurani pointed out, just make sure you use the surround (aka side surround) channels for the 4th and 5th speaker regardless of their ultimate placement.

"Surround back" is the term applied to the pair of rear surround speakers - as opposed to the pair of (side) surround speakers - in a 7.1-channel set-up.

In a 5.1-channel set-up, there's only one pair of surrounds (and no rear surrounds). Where you choose to locate the surround speakers - at the side of, slightly behind or directly behind the listening position - is different issue.

THX recommends "between 90° to 110° to each side and 2 feet or higher above the listener"...but life is full of compromises.