The "M" in the Formula for Slope

Nobody knows for sure why the letter M is to represent slope in the equation. Some mathematics professors have traced the use of it in equations to as far back as books from 1820. Descartes didn’t use the “m” (in this context), though Euler did.

Possible explanations:

-     Mathematician John Conway says: Perhaps it stands for “modulus of slope.” The term “modulus” has been known to be used for “the essential parameter of determining.”

-     It could stand for the French verb monter, which means “to go up,” “to ascend,” “to climb,” “to slope up,” etc.

-     Possibly from the French montaire (“mount”) or  montaigne (“hill”).

-     Possibly from mons, the Latin word for “mountain.”

-     M. Risi, a former author of books on mathematics, said: “In our system, the first letters of the alphabet, a, b , c ... represent the constants, the last letters, x, y, z represent the unknown variables, and the middle letters, m, n, p ... represents the parameters.  When we started the explanations of slope, it was in studying the first degree equation:  y = mx + b.  x and y were the variables, b was fixed and considered as a constant, and what was appended to the coefficient of x as its value varied. So it was a parameter and that is why we used m.”