why is a gooseneck trailer hitch, and the 5th wheel semi trailer hitch called a "5th Wheel"?

here is what the originals looked like, above, the mocked up prototype, below, a finished working replica for the 1914 model t

When Otto Neumann and August Fruehauf invented the semi-trailer in 1914, they created a version of a 5th wheel coupling which they used until 1916. Then they adopted an improved version of the fifth Wheel offered by the Martin Rocking Company in Massachusetts.

above and below, the Martin 5th wheel

which was improved on until it became the common and ordinary hitch you're used to seeing

http://www.singingwheels.com/the-5th-wheel-and-its-origins.html