Foam-Encased PowerEdge Bonnell System: Designed for maximum durability, reliability and comfort with a proven record of performance. Innerspring: Double-tempered, hourglass shaped heavy-gauge steel innerspring designed for durability and functions on a progressive rate of compression.Edge Support: High-density foam-encased rails deliver superior support for a firmer seating edge. Sleeping surface extends across the entire bed, right up to the mattress edge.