The King's English

In 1614, Queen Anne died without heir.  The powers that were in Britain, namely Parliament, had decided that after three terrible kings who were Catholic, that they didn't want a Catholic King again.  So they banned Catholics from the throne.  The nearest non-Catholic heir was George, Elector of Hanover, Anne's 4th cousin.  George wasn't really interested in being King of England and he spent very little time there, and probably never learned to speak much English.  But his court, and many of his new English subjects, tried to please him.  Among the many small gestures was mimicking his thick German accent.  This became known as speaking "The King's English" and portions of it remain in the English accent to this day. ("Elector" is a title similar to Duke, but in the Holy Roman Empire, was part of the process of "electing" the new Emperor)

For example, the German "R" sound is rolled, where prior to George, the English "R" had been hard, what is called "rhotic", like today's mainstream American accent.  George was apparently able to stop rolling his Rs but never quite make a true hard one, so Rs were dropped.   There is no "S" sound in German, but there is a double S, so many speakers of the Kings English have over-sibilant Ss.   The sound that Germans spell "Sch" is spelled in English "Sh".  So George confused the two, and words which had been pronounced "Sk" but spelled "Sch", such as "Schedule", came to be pronounced "Shedyule".  There's no "Th" sound in German but they spell what an English speaker would spell with a "T" with a Th.  Thus the river that runs through London became the "Tems", while retaining it's spelling "Thames".  (Interestingly, this seems to be a case of the sound being switched twice:  Rivers in the new world that were named for it, such as the one in Connecticut, are pronounced with a Th sound, but it appears that before the renaissance and the spelling firmed up, it had been pronounced "Tems".  But during the time the English were colonizing America, it was pronounced "Thames")

It became fashionable for a time in the New World to mimic the King's English, so things like R dropping became popular in a number of the more fashionable coastal cities.  But when the Americans rebelled against George's great grandson, George III, this became less fashionable.  But pieces of it remained in some American dialects.   The inland American dialect continued to move in it's own direction.   And much of England didn't really adopt it.

Conveniently for historians of American English, a great deal of research has been done on the "correct" way to pronounce Shakespeare. (Shakespeare and the earliest American colonies were contemporaneous) An English "RP" or "Public School" accent is quite wrong, and many of the rhymes and puns just don't work.  An American accent is still wrong, but less so.  I've always thought it really funny to hear an American doing Shakespeare, aping an RP accent. The modern accent that sounds nearest, to my ear, is Cornwall, where they still pronounce a lot of R, but even that is not right.