Support core, also referred to as inner core or poly core, is a support portion of the foam mattress. Where innerspring mattresses use steel coils for support, foam mattresses use high-density polyurethane foam or latex usually 3-8 inches high. High quality mattresses use high-density polyurethane core (6-8 inches) whereas lesser-quality products will either use thinner support layers (3-6”) or lower-density foam.