A dolly is a small trailer that can be coupled

Tow hitch

A tow hitch is a device attached to the chassis of a vehicle for towing or a towbar to an aircraft nose gear, or paired main gears....

 to a truck

Truck

A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...

 or trailer so as to support a semi-trailer

Semi-trailer

A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, a detachable front axle assembly known as a dolly, or the tail of another trailer...

. The dolly is equipped with a fifth wheel

Fifth wheel coupling

The fifth wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some recreational vehicles use a fifth wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle...

 to which the semi-trailer is coupled. This dolly needs its own rear lights and a registration plate

Vehicle registration plate

A vehicle registration plate is a metal or plastic plate attached to a motor vehicle or trailer for official identification purposes. The registration identifier is a numeric or alphanumeric code that uniquely identifies the vehicle within the issuing region's database...

.

A tow dolly is little more than two wheels, an axle and a tow-hitch, used to tow a Front-wheel drive

Front-wheel drive

Front-wheel drive is a form of engine/transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitudinal engine arrangement generally found in rear-wheel drive and...

 suspension car behind a recreational vehicle

Recreational vehicle

Recreational vehicle or RV is, in North America, the usual term for a Motor vehicle or trailer equipped with living space and amenities found in a home.-Features:...

 or other larger vehicle.

There are two basic types of dolly:

• Converter dolly, equipped with between one and three axles and designed to connect to a towbar on the rear of the truck or trailer in front. There are two variants of this:
  • An A-dolly has a single drawbar
    Drawbar (haulage)
    A drawbar is a solid coupling between a hauling vehicle and its hauled load. Drawbars are in common use with rail transport, road trailers, both large and small, industrial and recreational, and with agricultural equipment.-Agriculture:...
    
     with a centred coupling.
  • A C-dolly has two separate couplings side-by-side.
• Low loader
  Lowboy (trailer)
  A lowboy or dolly in Australia is a semi-trailer with two drops in deck height: one right after the gooseneck and one right before the wheels. This allows the deck to be extremely low compared with other trailers...
  
  dolly, equipped with a gooseneck type drawbar that attaches to the fifth-wheel coupling on the rear of a prime mover
  Tractor unit
  A tractor unit, prime mover , road tractor, or traction unit is a heavy-duty commercial vehicle within the large goods vehicle category, usually with a large displacement diesel engine, and several axles. The tractor unit serves as a method of moving trailers...
  
   to distribute the mass on the fifth wheel on the dolly between the prime mover and the wheels of the dolly. These are predominantly fitted with two axles.

Dollies are popular in Finland and Sweden, where the law allows longer vehicle combinations than in the rest of the European Union. By using a dolly, a semi-trailer can be attached as a regular trailer to the towing truck.

Australia

Converter dollies are used in road train

Road train

A road train or roadtrain is a trucking concept used in remote areas of Argentina, Australia, Mexico, the United States and Canada to move freight efficiently. The term "road train" is most often used in Australia. In the U.S. and Canada the terms "triples," "turnpike doubles" and "Rocky Mountain...

s in Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, most commonly with two or three axles and a hinged drawbar. They are also frequently referred to as road train dollies.

The C-dolly design is not allowed in Australia, as it prevents articulation between the dolly wheels and the axles of the truck or trailer in front of the dolly. Australian rules require articulation between axle groups.

Low-loader dollies- which present a kingpin rather than a drawbar coupling- are used with many low loaders to allow heavy cargo to be carried without overloading the wheels of the prime mover or the low loader.