We take our ABC's for granted, learning 26 letters in a precise order from our youngest days. When introduced to a second or third language later in life we may realize that even similar tongues to English contain slightly different alphabets--the Spanish ñ, the French ç--despite the fact that they evolved from the same roots. Historical variation in the English alphabet seems largely glossed over in contemporary education, but identifying some of the "missing letters" can help explain a few historical puzzles.
First, there's ampersand, considered the 27th letter of the English alphabet until about 150 years ago. It's name comes from its position at the end of the ABC's:
The word “ampersand” came many years later when “&” was actually part of the English alphabet. In the early 1800s, school children reciting their ABCs concluded the alphabet with the &. It would have been confusing to say “X, Y, Z, and.” Rather, the students said, “and per se and.” “Per se” means “by itself,” so the students were essentially saying, “X, Y, Z, and by itself and.” Over time, “and per se and” was slurred together into the word we use today: ampersand. When a word comes about from a mistaken pronunciation, it’s called a mondegreen.
Before the introduction of the Latin alphabet after the Roman conquest of Britain, Anglo-Saxon had an alphabet all its own known as furthorc. In the ensuing battle of cultural power politics Anglo-Saxon lost out. Collateral damage included the letter "thorn," pictured at right, pronounced with the hard "th" sound. It was replaced by the humble Y, always ready to do double duty in that ambiguous no-man's-land between consonants and vowels. This explains the anachronistic use of Y in titles like "Ye Olde English Shoppe"--it's just another spelling of "the."
On Friday we'll take a look at another missing letter, the long s (resembling "f"). For a sneak peek and a list of nine other extinct English letters, check out this article from MentalFloss (via @johndcook).