Until roughly the 16th century, speakers of all the varieties of the West Germanic languages from the mouth of the Rhine to the Alps had been accustomed to refer to their native speech as Dietsch, (Neder)duyts or some other cognate of theudisk. This let inevitably to confusion since similar terms referred to different languages. Therefore, in the 16th century, a differentiation took place. Owing to Dutch commercial and colonial rivalry in the 16th and 17th centuries, the English term came to refer exclusively to the Dutch. A notable exception is Pennsylvania Dutch, which is a West Central German variety called Deitsch by its speakers. Jersey Dutch, on the other hand, as spoken until the 1950s in New Jersey, is a Dutch-based creole.

In what century was a differentiation made to clear up confusion about West Germanic languages?