Robert Recorde invented the equals sign because he was tired of writing
In 1557 the Welsh mathematician Robert Recorde got fed up with writing out the words 'is equal to' again and again. So he drew two parallel lines of the same length, reasoning that no two things could be more equal. His invention then sat almost unused in print for 61 years.
Before the 16th century there was no shared symbol for equality. Writers used words such as aequales, esgale, ghelijck or gleich, or abbreviations like aeq, though Diophantus had shortened the Greek isos in his Arithmetica around AD 250. Recorde introduced his twin lines in The Whetstone of Witte, published a year before his death, and drew them much wider than the modern sign. He called them Gemowe lines, from the Latin word for twin.
The next printed use came only in 1618, in an anonymous appendix to an English translation of John Napier. In 1631 three influential English books, including works by Thomas Harriot and William Oughtred, adopted it, and later Isaac Newton and John Wallis helped carry it abroad. Continental Europe held out. Xylander's 1571 edition of Diophantus used two vertical bars, and René Descartes introduced his own symbol in La Géométrie in 1637; by 1675 his version was more popular in Europe, possibly because the two lines were also being used to mark differences.
Programming split the sign in two. FORTRAN I, designed in 1954, used it to assign values, so X = X + 2 simply adds two to X. ALGOL, designed in 1958, used it to test equality instead, a choice followed by Pascal and Ada. The language B introduced a doubled sign for testing equality, and C and its many descendants copied it. JavaScript's loose double-equals is so inconsistent that programmers are often advised to use the triple version instead.
The symbol has other lives too. It marks tone in several West African writing systems, stands for a double bond in chemistry and means Break Text in Morse code. Alberto Santos-Dumont placed a similar double hyphen between his surnames to honour his French father and Brazilian mother equally.
Source: Equals sign
