Class III Trailer Hitch Receivers Designed for trucks, vans and sport utility vehicles 2" x 2" receiver hitch opening Rated up to 6,000 lbs GTW with 600 lbs TW Allows for more gear to be carried or towed Works with all Class III accessories, such as bike racks, cargo carriers, and tube covers Class I and Class II (1-1/4") hitch accessories can also be used with an adapter Ball mounts are sold separately. Will work with hitch balls with 1" diameter shank
NOTE: Most manufacturers call any hitch with a 2" x 2" opening a Class III hitch. It is not uncommon to see a 2" hitch rated at 3,500 lbs GTW called a Class III hitch Available in square and round tube designs.

http://www.etrailer.com/faq_hitchclasses.aspx