Who invented the equals sign? Robert Recorde, in 1557

The equals sign (=) was invented by Welsh physician and mathematician Robert Recorde, who introduced it in his 1557 algebra book The Whetstone of Witte to avoid writing out “is equal to” again and again. Born in Tenby around 1510, Recorde wrote some of the first popular mathematics textbooks in English and introduced algebra to English readers.

He explained his choice in a single sentence, in his own spelling:

And to avoide the tediouse repetition of these woordes: is equalle to: I will sette as I doe often in woorke use, a paire of paralleles, of Gemowe (or twin) lines of one lengthe … bicause noe 2 thynges can be moare equalle.

Gemowe means twin. The book’s title was a joke as well: algebra was then called the cossic art, after cosa, the “thing” that stood for an unknown, and cos is Latin for whetstone. It was an algebra book to sharpen your wits on.

Did everyone adopt the equals sign at once?

No; the sign was not an instant standard. Some writers used two vertical bars, ||, for equality, and the abbreviation ae, from the Latin aequalis, stayed in wide use into the 1700s. Recorde’s twin lines won out only gradually.

Why did Recorde die in prison?

Nobody is quite sure. A former controller of the Bristol mint and surveyor of the Irish mines, he accused the powerful Earl of Pembroke of misconduct in 1556. Pembroke sued him for libel, won in January 1557 and was awarded £1,000, exactly the sum Recorde was himself owed, and had never been paid, for his Irish work. He died in the King’s Bench prison in Southwark in 1558, and his will was proved on 18 June.

The common story, repeated by MacTutor, is that he could not or would not pay the damages. The Dictionary of Welsh Biography finds no evidence he was pressed to pay, and suggests the Privy Council held him while it investigated the failed Irish silver mines. The money he was owed reached his estate only in 1570.