^Cajori, Florian. Origin and meanings of the signs + and -. A History of Mathematical Notations, Vol. 1. The Open Court Company, Publishers. 1928.
^Wright, D. Franklin; New, Bill D. Intermediate Algebra 4th. Thomson Learning. 2000: 1. The minus sign or bar, — , is thought to be derived from the habit of early scribes of using a bar to represent the letter m