Johnson's dictionaries did include a section of X but, because there were no words starting with X at the time, he wrote:
X is a letter, which, though found in Saxon words, begins no word in the English language.
That was the entire entry for that section.
Today, of course, there are many words, most of which are from Greek.
Xmas, X-mas or Christmas?
Christmas: Xmas and X-mas are informal forms.
X-ray or x-ray?
Always X-ray with a capital letter.
This page last updated: 15 November 2014
Jesse Karjalainen is the author of The Joy of English: 100 conversations about the English Language, Cannibal – the language and history of the discovery of the New World, and Roanoke – the language and history of Early Colonial America.