This CRL Framing Upper Channel is designed with a flange on one side for framing of sliding door openings. May be used in place of the D603 Upper Channel or D609 Upper Channel. The plastic lined upper track has proven most efficient for sliding glass doors because no "tolerance" is required at the top for clips or guides. This allows the glass door to enter the upper track 1/8" (3 mm) further. The glass is held vertically with a minimum of "play", and slides silently and smoothly.