Equal Mark
In Robert Recorde’s “Whetstone of Witte,” a treatise on algebra written about the year 1557, he says, “To avoide the tediouse repetition of these words, is equalle to, I will sette, as I doe often in worke use a pair of parallel lines of one lengthe, thus: =, because no two things can be more equalle.” This was the origin of this common arithmetical sign.