Rogues, to speak thus irreverently of the alphabet, I shall live to see you glad to serve old Q — to curl the wig of great S — adjust the dot of little i — stand behind the chair of X. Y. Z. — wear the livery of Etcetera — and ride behind the sulky of And-by-itself-and.
From Act I of Charles Lamb's Mr. H
If you were a schoolchild in the 19th century, the alphabet that you learned would have had 27 letters: all 26 letters of our current alphabet, plus the ampersand symbol.
Due to the awkwardness of ending a recitation of the alphabet with "W X Y Z and", it was traditional to instead say "W X Y and Z, and per se and", where per se, Latin for "by itself", means that &, standing by itself, represents "and". (Words with one-letter spellings, like A or I, were often orally spelt as "A per se" or "A per se A".) It was this process of alphabet recitation, and hurried enunciations of "and per se and" which spawned a variety of names for the & symbol which ultimately converged on "ampersand". & had been part of the alphabet going back to the days of Old English.
Other than standing for the conjunction "and", the ampersand also sometimes appears in the abbreviation &c, for et cetera. This is due to the origins of the & symbol in the first century A.D., when the Romans would write et (Latin for "and") in cursive in a run-together fashion which became a stand-alone written symbol.
So why do we no longer consider & to be part of the alphabet?
The leading theory is that it's because of that alphabet song, the one that goes
A B C D E F GMany incorrectly believe that this is based on a tune by Mozart. While Mozart wrote variations on this theme at the age of 25 [see Köchel listing K. 265], the original melody that inspired him was a French folk song called "Ah! vous dirai-je, Maman" which eventually served as the music for Twinkle, Twinkle, Little Star. In 1835, the alphabet song was copyrighted under the name "The A.B.C., a German air with variations for the flute with an easy accompaniment for the piano forte", so it does seem like Mozart was responsible for popularizing the melody.
H I J K LMNOP
Q R S, T U V
W X, Y and Z.
Now I know my A B Cs.
Next time won't you sing with me?
It turns out that that song has influence beyond the ousting of the ampersand. Historically, it has been mainly in the U.S. that Z has been pronounced zee; pretty much everywhere else they say zed. But a quick look at the rhyming scheme of the alphabet song, shows that the zee pronunciation works better. And apparently a lot of children who learn English outside of the U.S. are still exposed to this alphabet song through American children's programming, like Sesame Street. Teachers in England reportedly have to correct kindergarteners who enter school singing the alphabet song in an American accent, right down to the zee.
I leave you with this quote from Steven Wright:
Why is the alphabet in that order? Is it because of that song? The guy who wrote that song wrote everything.