It's a stack spacer. It's a top cap. It's both! The Ultralight stem cap is designed specifically for carbon steerers that require that the steer tube extend above the stem.1 1/8" top (straight 1 1/8" or 1 1/2" to 1 1/8" taper) steerer tubes onlyLight-weight and durable 6061-T6 aluminumItem SpecificationsColorEspressoDefined ColorBrown Wolf Tooth Ultralight Stem Cap with Integrated Spacer Headset Top Cap UPC: 810006806441