The Sammons Preston Solid Seat Insert is designed for use in wheelchairs with sling seats. This semi-rigid closed-cell foam insert provides a firm, flat seating surface in sling seats. This surface also helps improve posture by helping to balance the pelvis and distribute weight over the hips and thighs. This insert is available in standard, narrow, or relaxed-fit styles. DIMENSIONS (WxD): The standard model is 18 x 16 inches, the narrow model is 16 x 16 inches, and the relaxed-fit model is 20 x 16 inches.