EXT or RX Stringer's are pulltruded composite fibre tubes. (Resin Extruded) These hollow composite rod's that is added to the core to provide more structural strength. Bodyboard core's are drilled out usually through the middle & centre of the board & the stringer added before the core is laminated. Bigger boards or boards that require more strength can have 2 or 3 stringer's to add stiffness. EXT Stringer's are usually added to mid range model's.