The correct length of the seat is 6.5 centimeters from the front edge of the cushion behind the knee, about 4 finger widths after the disabled person sits down. If the seat is too long, it will push against the back of the knee and compress the blood vessels and nerve tissues, and abrade the skin. If the seat is too short, the pressure on the buttocks increases, causing discomfort, pain, soft tissue damage and pressure sores.