Back sleepers should position it under their knees to maintain natural spinal alignment, or behind the lower back for extra lumbar support.
Side sleepers benefit most from placing the bolster between their legs to keep hips aligned and reduce back strain, though it can also be hugged for upper body comfort or used as an elevated head pillow.
While not suggested for most stomach sleepers due to potential neck strain, they can place a thin bolster under their pelvis to help maintain proper spine alignment; however, this sleeping position generally isn't recommended for optimal spinal health.