A term often misused in the industry. It is a measure of how well a mattress supports and maintains the neutral alignment of the spine and joints. The primary support or deeper layers are mainly responsible for stopping the heavier pelvic girdle from sinking down too far into the mattress relative to the other lighter parts of the body while the upper or comfort layers provide the secondary or lighter support which fills in the gaps in the sleeping profile (like the waist or lower back) and helps to maintain the inward curves of the body and the spine in natural alignment.