The Manfrotto 131DDB Tripod Accessory Arm for Four Heads mounts onto the center columns of tripods with 3/8" threads. Its platforms feature 3/8" screws, which let you mount a standard tripod head on either or both sides at 90° perpendicular angles. Its horizontal bar also features 3/8" screws on both ends so that you can mount tripod heads from the sides.