YES 40 FS (1-3/4” x 4”) is a center set, flush glazed storefront framing system designed primarily for 1/4” glass or infill panels 1/4” to 3/8” thick.This architectural commercial storefront system will accommodate YKK AP’s hinged mullion, as well as our flexible mullion; these mullions allow more creative freedom where curves and angles become a functional part of the design.