Your back muscles connect to your butt muscles, which then connect to your hamstrings, etc … Taking tension off your legs will then relax your back slightly. So, if you’re laying on your back and put something under your knees, for example, this will flex the knees, decreasing leg and back tension. Your hamstring muscles become shortened and relaxed lowering tension in your gluts and therefore your lower back. On your side, a pillow between the knees will keep your legs and pelvis level. Think about being on your side if one leg flops well over the other, your pelvis is forced into rotation. The name of the game is to keep pressure off of nerves, and keeping the spine in alignment does this. “How does a Pillow Under or Between my Knees Help my Back?” CT Chiropractor Blog Post - October One more thing to keep in mind, just like the muscles, but is true for the spine as well, is that it's completely connected. For example, if your cervical curve (neck alignment) is just right then your thoracic curvature maintains better alignment. The lumber spine then can stay in good position. That being said, it's a good idea to use a chiropractic neck pillow too. These are a couple tricks with a pillow to help keep your back relaxed and without nerve pressure, therefore without pain. Happy sleeping!