The equal sign (=) is a relatively “new” invention in mathematics. It was first used by British mathematician Robert Recorde (1510-1558; also seen erroneously as Record), in his book The Whetstone of Witte (1557), the first algebra book introduced in England. In it, he justifies using two parallel line segments “…bicause noe 2 thynges can be moare equalle (sic)…” (“because no two things can be more equal than parallel lines”). It was not an immediately popular symbol, though, with mathematicians continuing to use a range of symbols for equal, including two vertical parallel lines ( II ) used by Wilhelm Xylander in 1575, and ae or oe (both from the word aequalis, the Latin for “equal”). But for the most part, the word “equal” was written in an equation until around 1600, when Recorde’s symbol became more readily accepted, and it continues to be so today.