Why is a fifth-wheel called a fifth-wheel?

I’d guess it is because a 5th wheel on a 4 wheel vehicle, such as a car, isn’t really necessary & doesn’t serve a useful purpose. Would also not have a “matching partner” like the other 4 wheels.

My guess is because the truck acts as the fifth wheel.

Are we all talking about the same fifth wheel? I agree with @JOE, based on a fifth wheel being a large, dual axle trailer that attaches to a hitch usually placed inside the bed of a pickup truck, in which that attachment to the truck operates as the fifth wheel, since the other four are too far back to hold up the trailer on their own. @BraveWarrior seems to be talking about calling a person a fifth wheel in a social setting, and I have no idea what @ElleBee means about it being round like a wheel?

Sorry, to clarify, I mean the truck trailer sort of deal. First question asked: bare with me ;)

It comes after 4th wheel.

@BraveWarrior I agree with you.

I had to look up what a fifth-wheel is. So I went straight to Wikipedia for a picture, and instead I found this explanation: The term fifth wheel comes from a similar coupling used on four-wheel horse-drawn carriages and wagons. The device allowed the front axle assembly to pivot in the horizontal plane, to improve turning. (source)

Sorry, guess I should have looked at the question’s “Topics” before I answered. :)

In the social setting (being an extra person along on a date) the correct term is third wheel. It has been mistakenly renamed fifth wheel because there appears to have been a television show by that name.
I just love mixed metaphors.

@breedmitch Ah, yes, you’re right… guess “fifth wheel” would be an extra person on a double-date! :)

The fifth wheel is that notched round plate you see above the rear tires of a semi cab for attaching trailers or in the bed of a pick-up truck for attaching (usually) “camping” trailers (as opposed to a ball like bumper hitch). See ya….Gary/wtf