I was initially going to write about “betwixt,” as the third item in a trilogy started by “whilst” and “unbeknownst.” It is true that this synonym for “between” has traditionally been more popular in Britain than the U.S.
And it’s true that it seems to be popping up in America a bit more frequently as a cute and old-timey variation, especially in a mystical or fantastical context, as in the title of this book by a Maryland-born author.
The most common use, in my experience, remains the phrase “betwixt and between,” which, of course, means “between and between.” Google Ngram Viewer shows that after it came into prominence in the mid-1800s, Britain and the U.S. alternated in liking it more, with a long period in the mid-20th century with Britain in the lead, followed by equality since about 1980.
But “betwixt” ultimately isn’t popular or interesting enough for me to say anything more about it. So I’m just going to leave it there.
notoneoffbritishisms.com 
“Betwixt and between” is very familiar–my mother and aunt used it frequently in reference to emotions, states of mind, and the like, but they used it with an implied ‘Look how old-fashioned we are being! ‘ to rob it of any taint of
unhappiness.
‘Betwixt’ is a ‘fossil word’ – broadly obsolete but which remains in currency only when part of an idiom which is still in use.
e.g kith and kin, beck and call
Was Nathaniel Hawthorne wrong in frequenting ‘betwixt’ as much as he did in The Scarlet Letter, a tale set in the 1700s?