The ball mount or accessory in the vehicle's receiver tube - it replaces the plain pin and clip that anybody can pull out by hand.

A standard ball mount is retained by a pin through the receiver with an R-clip on the end.

That clip is removable without any tool, so the ball mount is secured only by nobody having bothered.

Adjustable drop mounts, bike carriers and cargo platforms are all worth real money and are all held the same way.

A locking pin replaces the clip with a key-operated mechanism, so removal needs the key.

It secures the accessory to the vehicle; it does nothing about the trailer, which is what a coupler lock is for.

Anti-rattle designs do, and it is often the main reason people buy one - the rattle comes from clearance between the ball mount and the receiver tube.

The receiver tube is deliberately made slightly larger than the ball mount so it can be inserted easily.

That clearance lets the mount move, producing a clunk on take-up and a rattle on rough surfaces.

It also wears both the tube and the mount over time, and the movement gets worse as it wears.

Anti-rattle locking pins take up the slack with a threaded wedge, an offset pin or a clamping collar.

A plain locking pin secures without curing the rattle, so if the noise is the complaint, the specification has to say anti-rattle.

By the receiver tube size, which determines the pin diameter - the two common receiver sizes take different pins and they are not interchangeable.

Receivers come in standard square tube sizes, and each has a corresponding pin diameter.

A pin for a smaller receiver will pass through a larger one's holes with excessive clearance, which is unsafe as well as rattly.

A pin for a larger receiver will not fit the smaller one at all.

Pin length also matters where an accessory is wider than a plain ball mount or where a sleeve or reducer is fitted.

Measure the receiver rather than assuming from the vehicle, particularly where a hitch was fitted aftermarket.

They sit directly in road spray and salt - lubricate the barrel and the pin, and use a cover where one is supplied.

The pin passes through the receiver at the lowest, most exposed point of the vehicle.

Salt, grit and water reach it constantly, and the pin surface corrodes in the tube where it cannot be seen.

A corroded pin can become genuinely difficult to withdraw, and a seized lock barrel makes it impossible.

Use a lock-specific lubricant on the barrel and a light anti-corrosion film on the pin shaft, and fit the weather cap if one is supplied.

Remove and re-lubricate periodically rather than leaving it in place for years, and treat it as a wear item to be replaced when it stops working smoothly.

A properly rated one is - but rating matters, because the pin carries towing loads as well as providing security.

The pin is a structural part of the towing connection, transferring load between the ball mount and the receiver.

It must be rated for the towing capacity in use, not merely be a lock that happens to fit.

Reputable locking pins state a load rating and are made from suitable material; unbranded ones frequently do not.

A pin that fails in service releases the ball mount and the trailer with it, which is why this is not a place for an unrated part.

Check the rating against the vehicle's and hitch's towing capacity, and replace any pin that is bent, deeply corroded or damaged.

On vehicles carrying removable ball mounts or accessories, yes - the losses are steady and rarely recorded as theft.

A missing ball mount usually gets reported as somebody having borrowed it, so the pattern is invisible in the maintenance record.

The cost of a locking pin is a fraction of a single adjustable drop mount or bike carrier.

It also removes the rattle complaint, which reduces the number of vehicles coming in for a noise nobody can find.

Keyed-alike sets across the fleet avoid the key management problem entirely.

Include the pin in the trailer and towing inspection so seized or damaged ones are found before they matter.

No - it secures what is inserted into the receiver. A bolt-on towbar or a detachable neck has its own arrangements.

The receiver tube is part of the towbar structure and is bolted to the vehicle.

A locking pin only secures the removable item inside it.

Detachable and swan-neck towbars have their own integral locking mechanism, usually with a barrel and key in the neck itself.

Those mechanisms are both security devices and safety-critical latches, and their correct engagement should be confirmed visually before every tow.

Fixed towbars are bolted on and are effectively secured by being awkward to remove, which in practice is sufficient.