The letter x has several meanings besides being the 24th letter of the alphabet. On its own and usually as a capital letter, X is the letter of the Greek alphabet equivalent to Ch, the first two letters of Christ. Hence, we get Xmas as a common abbreviation of Christmas. In some Christian liturgies, the abbreviation of Christ is written in Greek as XP, where P is in fact the Greek R. In algebra, x symbolises an unknown or variable quantity. X is also the symbol for a kiss, which we still add to cards and correspondence. This custom derived from The Middle Ages when illiterate people had to sign their name with a cross, as in the Holy Cross, which they then kissed in affirmation. From the late 18th century, signing with a cross, as long as it was witnessed, constituted a legal document. See also X rays.