Which Welsh mathematician introduced the equals sign?

Robert Recorde, in 1557, who wrote that no two things could be more equal than a pair of parallel lines - and used a pair very much longer than the modern sign, which took about a century to catch on against the alternative of writing the word out.

He was doing something larger than inventing a symbol. Recorde wrote the first English textbooks of arithmetic, geometry and algebra, in English rather than Latin, in dialogue form between a master and a scholar, aimed at merchants and surveyors rather than universities. He also introduced the plus and minus signs to English readers. Before this, mathematics in Britain was written in words and in Latin, which put it out of reach of everyone who might use it.

It ended badly. Recorde, who was also physician to Edward VI and comptroller of the mints, sued a powerful earl for misconduct, lost a countersuit for libel, could not pay the damages, and died in a debtors' prison in 1558. The book containing the equals sign was called The Whetstone of Witte, a pun: the Latin for an unknown quantity in algebra was cosa, the thing, giving the cossic art, and a whetstone sharpens.