If you step into most people's homes and look at their 5.1 audio setup you're going to notice some things which are common to them all.
5.1.2 Speaker placement Audioholics Home Theater Forums
The Art of Speaker Placement for Home Theater and Stereo – SVS
Setting up speakers correctly: The 2.0 or stereo system - sonoro
Soundoctor - Surround Sound
Placing Your Speakers Right For The Best Sound Experience
Surround Sound Speaker Placement
5.1 or 7.1: Which sound system is right for you?
Do I Need a 5.1 Surround Sound System for My Computer?
audio - If I add two arbitrary speaker to an existing 5.1 sound system, can I get the equivalent of 7.1 sound? - Super User
TX-NR6100 - Speaker Placement/Multichannel surround Layouts – Onkyo Product Support
Home Theater: Surround Sound Speaker Placement
How to Set Up a 7.1 Surround Sound Speaker System