Excerpt from Spell It Out by David Crystal, plus links to reviews, author biography & more

This letter was introduced by Norman scribes in the 11th century as a means of representing the sound /w/, replacing the runic letter which had been used in Old English. Although its shape is a ligature of two vs, its name is 'double u', reflecting the state of affairs in Middle English when v and u were interchangeable. The lower-case letter is a smaller version of the capital.

X,x

X emerged in the Greek alphabet, derived from an earlier Semitic sibilant letter. It came into Latin with the value of /ks/, and was used in Old English typically as a variant spelling of cs. The lower-case letter is a smaller version of the capital.

Y,y

Y is a Greek adaptation of a Semitic symbol. In Roman times, it was borrowed to help transcribe Greek loanwords into Latin. The rounded lowercase letter developed as part of handwriting, enabling scribes to write it in a single movement, The trunk of the letter was placed below the line, and moved to the right to enable a smoother link to be made with the following letter.

Z,z

Z appeared in the Semitic and Greek alphabets, and although it was not needed for Latin, the Romans later borrowed the symbol to help transcribe Greek loanwords, making it the last item in their alphabet. The lower-case form is a smaller version of the capital.

From Spell it Out by David Crystal. Copyright © 2013 by the author and reprinted by permission of St. Martin's Press, LLC.