This is an old question and there have already been lots of historians, linked to the mathematical sciences, who have tried to find an answer. However, there isn’t still a consensus within the scientific community concerning the origins of the letter m used to represent the slope of a straight line.

Some of them believed that its origin goes back to the French word “monter”, which means to go up. However, lots of them say it is just an urban myth, since Descartes, who was a French man, did not use that letter to represent the slope of the straight line. There are others who argue that letter `m` has its roots in the English term “modulus of slope”, according to which “modulus”(which is used in the sense of “Used number to measure) was lost as time passed by and just the term “slope” was left in order to mean inclination. Other mathematicians even argue that the first letters of the alphabet `a`, `b`, `c` were used to represent mathematical constants while the last letters `x`, `y`, `z` were used to represent variables whose values are unknown. Finally, the letters that are in the middle – `m`, `n`, `p` – were used to present parameters. When we start the study of first degree equations: `y = mx + b`, the letters `x` and `y` are the variables, `b` is fixed and it can be considered a constant and `m` is a parameter whose value varies and it is used to measure the slope of the straight line.

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In conclusion, although there are several versions about its appearance, nobody can say for sure what its origins are. We can also take notice that not all the countries use the letter m to represent the slope of a straight line. For instance, in Sweden, Norway, Russia and Australia, people use letter `k`, whose probable roots come from the word “koefficient”, this is, coefficient. It also makes sense, since the slope can be considered as being the ratio between the differences of the “height” of two points. If you want to know more about how you can calculate the value of the slope, have a look at the given answer to the question: What is the slope of a straight line?




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