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Did you ever wonder why we say the alphabet as: ABCDEFG...WXY and Z?
There are 26 letters to the English alphabet, with two of the letters -- "A" and "I" -- themselves also constituting words. But as seen above, another character -- the ampersand (&) -- also, at times, was included among the current 26. And amazingly, the word "ampersand" is probably a byproduct of the symbol's inclusion.
The picture above is from a 1863 book called "The Dixie Primer, For The Little Folks," available here in its entirety -- a book which like many around even today, aimed at teaching children their ABCs and some basic words and phonics. Notably, the ampersand is included in the alphabet, just next to the Z and ending the entire set. While not necessarily the standard usage, it was not terribly uncommon either to include the ampersand here -- it had been there for centuries.
The ampersand was developed along with the rest of the language, reaching back to the 1st century, when Romans would occasionally combine the letters "E" and "T" into a similar symbol, representing the word "et" meaning "and." It was included in the Old English alphabet which was still in use into medieval times. When Old English was discarded in favor of the modern English were are now familiar with, the ampersand maintained its status of "member of the alphabet" (to coin a phrase) to a degree, with some regions and dialects opting to include it until the mid-1800s.
Except that it was not yet called an ampersand. The & sign was, rather, referred to simply as "and" -- which made reciting the alphabet awkward. As Dictionary.com notes, it was (and is) odd to say "X Y Z and." So, they didn't. Instead, our lexicon developed another saying: "X, Y, and Z, and by itself, 'and'" -- but instead of saying "by itself," the Latin phrase per se came into favor. The result? "And per se, and," or, muttered quickly by a disinterested student, "ampersand."
Why the inclusion of the ampersand in the alphabet fell out of use is anyone's guess, but there is a good chance that credit goes to the ABC song we are most all familiar with -- that is, the one which shares its tune with Twinkle, Twinkle, Little Star (and borrows from Mozart's Ah vous dirai-je, Maman). That song was copyrighted in 1835, around the time that the ampersand started falling out of favor with the rest of the ABCs.