Which mathematician is credited with introducing the equals sign in 1557?

Robert Recorde is credited with introducing the equals sign in 1557.

Recorde was a Welsh physician and mathematician who wrote influential English-language textbooks. In his book The Whetstone of Witte, published in 1557, he used two parallel lines to represent equality. He explained that no two things could be more equal than two straight lines of the same length.

The symbol did not become universal immediately. Earlier mathematical writing often expressed equality with words or abbreviations, and printers and mathematicians used several competing signs. Over time, Recorde’s paired lines became the standard equals sign used in algebra and arithmetic.

Recorde’s books helped make mathematical ideas more accessible to English readers. The Whetstone of Witte is also associated with one of the earliest printed uses of the plus and minus signs in English mathematical writing. The modern symbol is usually written as two short horizontal strokes, although Recorde’s original lines were longer.