California Vehicle Code
Sec. § 225

An “auxiliary dolly” is a vehicle, not designed for carrying persons or property on its own structure, which is so constructed and used in conjunction with a semitrailer as to support a portion of the weight of the semitrailer and any load thereon, but not permanently attached to the semitrailer, although a part of the weight of such dolly may rest on another vehicle.
Last accessed
Jun. 6, 2016