HERE’S MY BEST ANSWER AS TO WHY car companies other than General Motors, Ford, and Chrysler were called “independents.” The “Big Three” consisted of several makes, while the independents, at least before the mergers, consisted of just a single make. For example, in the 1950s every car Hudson made was a Hudson: Hudson Commodore, Hudson Hornet, Hudson Jet, etc. Likewise, Studebaker President, Studebaker Commander, Studebaker Hawk, and so on.
On the other hand, there was no such thing as a Ford Mercury, Ford Lincoln, Chrysler Plymouth, or Chrysler Dodge. And no car was a General Motors anything.
Eventually AMC tried to differentiate their models as makes, as Rambler, Ambassador and American rather than Rambler, Rambler Ambassador and Rambler American, although the latter nomenclature was widely used by the public. And…