Which mathematical symbol did Robert Recorde invent?

Robert Recorde was a Welsh mathematician and physician, born about 1512, the second son of Thomas and Rose Recorde of Tenby, Pembrokeshire.

Recorde entered the University of Oxford about 1525, and was elected a Fellow of All Souls College in 1531. Having adopted medicine as a profession, he went to the University of Cambridge to take the degree of M.D. in 1545. He afterwards returned to Oxford, where he publicly taught mathematics, as he had done prior to going to Cambridge. He later went to London and acted as physician to King Edward VI and to Queen Mary, to whom some of his books are dedicated. He was also controller of the Royal Mint and served as Comptroller of Mines and Monies in Ireland.

Robert Recorde invented the “=” symbol, now universally accepted in mathematics for equality. The original form of the symbol was much wider than the present form as is shown by its first appearance in Recorde’s publication "The Whetstone of Witte" (1557). In that book Recorde explains his design of the "Gemowe lines" (meaning “twin lines”, from the Latin “gemellus”): “And to avoid the tedious repetition of these words: ‘is equal to’ I will set as I do often in work use, a pair of parallels, or duplicate lines of one (the same) length, thus: ‘=’, because no 2 things can be more equal.”

Recorde came to a sad end. After being sued for defamation by a political enemy, he was arrested for debt and died in the King's Bench Prison, Southwark, in mid-June 1558.