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The signs that we use in arithmetic are known to all, but their origin is not so familiar.

The sign =, meaning equal to, was first used by [Welshman] Robert Recorde, of All Souls' College, Oxford, in 1531. To save himself the trouble of writing the words "equal to" again and again, he drew two little lines equal to one another. [He said "Noe 2 thynges can be moare equalle” than parallel lines. His symbol appears to be 5 times the current length of the equal sign, as we can see in The Whetstone of Witte by Recorde, below.]

Recorde equal sign
Image source with text

The sign for addition (+) is really a carelessly made p, from plus, the Latin word for more.

The —, for subtraction, also comes from a shortened Latin word, minus, meaning "less than", which was written m n s, with a horizontal stroke on top to show that it had been shortened. Then the letters were omitted, and the stroke only written.