Placing a pillow between the legs can help maintain a more neutral pelvis and lower-back alignment during side sleeping. When the knees touch, the top leg may fall forward and downward, driving hip internal rotation and increasing lumbar torsion—often felt as morning low-back discomfort. A thin pillow between the knees keeps the legs more parallel and supports pelvic balance. This is commonly used to improve comfort in people with low-back pain, hip sensitivity, and during pregnancy. It is not a “cure,” but a supportive strategy to reduce mechanical load. If the pillow is too thick, it may over-abduct the hip; the ideal thickness should be individualized.