Stearns & Foster High Profile 9 in. Box Spring Twin XL 39 in. by Stearns & Foster $33999 High profile 9 in. box spring for twin XL size (39 in.) mattress SOLID BASE: Supports the mattress with a solid flat surface to help extend the mattress durability and comfort. SUPERIOR CONSTRUCTION: 2 in x 4 in components prevent the frame from shifting over time. Double centre supports prevent sagging, and structured corners provide protection to each corner. DISTINCT DESIGN: Specially designed to enrich the performance of Stearns & Foster mattresses. Complementing colours to pair with any mattress. Available in a choice of Low or High profile. COMPATIBILITY: Our foundation is compatible with any mattress on the market today. It is important to note that federal flammability standards are based on a set of the same brand.