Plus-minus sign
On Windows, hold Alt and type 0177 or 241 on the number pad, then let go of Alt.
Copy it
About the plus-minus sign
The plus-minus sign does two jobs. It gives a tolerance, as in 12 ± 0.5 cm, and it packs two answers into one line: if x² = 9, then x = ±3.
Albert Girard, a French mathematician, wrote an early form in 1626 together with the French word ou, “or”. The modern form appears in print in 1631, in Clavis Mathematicae by the English mathematician William Oughtred.