It can take the form of a tow-ball to allow swivelling and articulation of a trailer, or a tow pin and jaw with a trailer loop - often used for large or agricultural vehicles where slack in the pivot pin allows the same movements. Another category is the towing pintle used on military vehicles worldwide.
In the U.S. the vehicle attachment is known as the trailer hitch. Trailer hitches come in two main configurations: receiver type and fixed-drawbar type. Receiver-type hitches consist of a portion that mounts to the frame of the vehicle that has a rearward facing opening that accepts removable ball mounts, hitch bike racks, cargo carriers, or other hitch mounted accessories. Fixed-drawbar hitches are typically built as one piece, have an integrated hole for the trailer ball, and are generally not compatible with aftermarket hitch accessories.
A trailer hitch typically bolts to the chassis of the vehicle. In the U.S. there are a few common classes: I, II, III, IV, and V that are defined by the SAE.
Receiver-type hitches are typically offered with a square receiver opening of 1.25 inches / 31.7 mm x 1.25 inches (for Class I/II) or 2 inches / 50.1 mm x 2 inches (for Class III/IV/V). Some Class IV/V hitches are available in 2.5 inch / 63.5 mm x 2.5 inch opening sizes.
The trailer tongue (U.S.) or coupling (non U.S.) slips over a tow-ball. Tow-balls come in various sizes depending on the load they carry and the country of operation:
Outside the U.S. the vehicle mounting for the tow-ball is called the tow-bracket. The mounting points for all recent passenger vehicles are defined by the vehicle manufacturer and the tow-bracket manufacturer must use these mount points and prove the efficacy of their bracket for each vehicle by a full rig-based fatigue test.
In order to tow safely the correct combination of vehicle and trailer must be combined with correct loading horizontally and vertically on the tow-ball. Advice should be taken (see references) to avoid problems.
In North America the ball attaches to a ballmount. Receiver-type hitches use removable ball mounts, whereas the fixed drawbar type hitches have integrated ball mounts. The ballmount must match the US hitch class. The ballmount for a receiver-type hitch is a rectangular bar that fits into a receiver attached to the vehicle. Removable ball mounts are offered with varying rise or drop to accommodate variations in the height of the vehicle and trailer to provide for level towing.
The ISO standard tow-ball is 50mm in diameter and conforms to a standard BS AU 113b. The ISO standard has been adopted in most of the world outside North America.
There are two main categories of ISO tow-ball - the flange fitting and the swan-neck which has an extended neck fitting into the tow-bracket. Swan-neck tow-balls are often removable to avoid the inconvenience of a tow-ball protruding from the vehicle when not required. Some manufacturers are introducing retractable tow-balls as an option.
Across Europe around 25% of the vehicle park have tow-balls fitted - but there are distinct regional variations with Benelux and Scandinavia having 70 to 90% of vehicles with tow-balls.