Diamond Shape Base: The RAM universal plate is 1-1/4 x 2-1/2 diamond plate with two holes 1.912 from center to center. This base plate will attach to any of the RAM composite holders and industry standard AMPS hole pattern. Round Base: 2- 7/16 diameter base. Rail Base with Zinc coated U-bolt: Accommodates handlebars from .75 to 1.25 O.D. Angled Base with 9mm Hole: Fits well to mirror frame. Straight Base with 11mm Hole: Fits well on many handlebar pinch bolts with a washer.Base is 5mm thick. Sport Bike Stem Base: Designed to fit into the cylindrical bore wall of the fork stems.