Almost no one seems to ask about the b, which is even more curious to me.

Interestingly, m for slope has led to more mis-history speculation in classrooms than any other topic, with the possible exception of the life of that Great American-Indian Mathematician, Chief Soh Cah Toa.

Here is what I have found about the m for slope history.

Slope is derived from the proto European root slupan (another etymology site uses sleubh) for slip . The relation seems to be to the level or ground slipping away as you go forward. The root is also the progenitor of sleeve (the arm slips into it) and, by dropping the s in front we get lubricate and lubricious (a word describing a person who is "slick", or even "slimy").

Many variations of where the idea of M for slope originated seem to be mostly myth. One of the most common is that the letter was used by Descarte because it was the first letter of some French word or another that related. In a post to the AP Stats discussion list some years ago, Hector Hirigoyen shared the following story:

I was told by Mary Dolciani herself, that the SMSG group "decided "to use y=mx+b because of the French (Descartes, I presume)-"montant"; I found it strange because the "logical word" would be "pente"(which is slope (and the standard term in Spanish is pendiente, which matches this). However, several years ago, while visiting a French high school, I noticed the teacher used y=sx+b. I inquired, and she said because of the "American" word "slope." I believe they are using ax+b for the most part these days.

A similar story was shared by Paul Foerster:

I can confirm Hector Hirigoyen's statement about the origin of "m" for slope. Hector was in the audience when I did a presentation at a math teachers' meeting in Florida about 1986 or 1987. His point was that "m" came from the French "montant," meaning "the rise." Because he taught AP French as well as mathematics, it seemed like a logical conclusion.

I was so thrilled with this new knowledge that I told it to my BC Calculus class when i returned to San Antonio. To add to the credibility, I asked Cassandra Stapfer, my exchange student from Paris, to confirm that this was correct. Cassandra said, "montant does mean 'the rise,' but in France we use 's' for 'slope'."So much for my brilliant discovery!

Here are several other clips from postings about the topic on a discussion group about math history.

In his "Earliest Uses of Symbols from Geometry" web page, ... Jeff Miller gathered the following information: Slope. The earliest known use of m for slope is an 1844 British text by Matthew O'Brien entitled _A Treatise on Plane Co-Ordinate Geometry_ [V. Frederick Rickey]. George Salmon (1819-1904), an Irish mathematician, used y = mx + b in his _A Treatise on Conic Sections_, which was published in several editions beginning in 1848. Salmon referred in several places to O'Brien's Conic Sections and it may be that he adopted O'Brien's notation.

According to Erland Gadde, in Swedish textbooks the equation is usually written as y = kx + m. He writes that the technical Swedish word for "slope" is "riktningskoefficient", which literally means "direction coefficient," and he supposes k comes from "koefficient."

According to Dick Klingens, in the Netherlands the equation is usually written as y = ax + b or px + q or mx + n. He writes that the Dutch word for "slope" is "richtingscoefficient", which also means "direction coefficient." In Austria k is used for the slope, and d for the y-intercept. In Uruguay the equation is usually written as y = ax + b or y = mx + n, and the "slope" is called "pendiente", coeficiente angular", or "parametro de direccion".

It is not known why the letter m was chosen for slope; the choice may have been arbitrary. John Conway has suggested m could stand for "modulus of slope." One high school algebra textbook says the reason for m is unknown, but remarks that it is interesting that the French word for "to climb" is monter. However, there is no evidence to make any such connection. Descartes, who was French, did not use m. In _Mathematical Circles Revisited_ (1971) mathematics historian Howard W. Eves suggests "it just happened."

Jeff Miller's web site cited above now has updated the earliest use of m for slope.

The earliest known use of m for slope appears in Vincenzo Riccati?s memoir De methodo Hermanni ad locos geometricos resolvendos, which is chapter XII of the first part of his book Vincentii Riccati Opusculorum ad res Physica, Mathematicas pertinentium (1757):

Propositio prima. Aequationes primi gradus construere. Ut Hermanni methodo utamor, danda est aequationi hujusmodi forma y = mx + n, quod semper fieri posse certum est. (p. 151)

The reference is to the Swiss mathematician Jacob Hermann (1678-1733). This use of m was found by Dr. Sandro Caparrini of the Department of Mathematics at the University of Torino.

I just checked Jeff's site again and found the following list of notes from his readers:

In 1830, Traite Elementaire D'Arithmetique, Al'Usage De L'Ecole Centrale des Quatre-Nations: Par S.F. LaCroix, Dix-Huitieme Edition has y = ax + b [Karen Dee Michalowicz].

Another use of m occurs in 1842 in An Elementary Treatise on the Differential Calculus by Rev. Matthew O'Brien, from the bottom of page 1: "Thus in the general equation to a right line, namely y = mx + c, if we suppose the line..." [Dave Cohen].

O'Brien used m for slope again in 1844 in A Treatise on Plane Co-Ordinate Geometry [V. Frederick Rickey].

George Salmon (1819-1904), an Irish mathematician, used y = mx + b in his A Treatise on Conic Sections, which was published in several editions beginning in 1848. Salmon referred in several places to O'Brien's Conic Sections and it may be that he adopted O'Brien's notation. Salmon used a to denote the x-intercept, and gave the equation (x/a) + (y/b) = 1 [David Wilkins].

Karen Dee Michalowicz has found an 1848 British analytic geometry text which has y = mx + h.

The 1855 edition of Isaac Todhunter's Treatise on Plane Co-Ordinate Geometry has y = mx + c [Dave Cohen].

In 1891, Differential and Integral Calculus by George A. Osborne has y - y' = m(x - x').

In Webster's New International Dictionary (1909), the "slope form" is y = sx + b.

In 1921, in An Introduction to Mathematical Analysis by Frank Loxley Griffin, the equation is written y = lx + k.

In Analytic Geometry (1924) by Arthur M. Harding and George W. Mullins, the "slope-intercept form" is y = mx + b.

In A Brief Course in Advanced Algebra by Buchanan and others (1937), the "slope form" is y = mx + k.

According to Erland Gadde, in Swedish textbooks the equation is usually written as y = kx + m. He writes that the technical Swedish word for "slope" is "riktningskoefficient", which literally means "direction coefficient," and he supposes k comes from "koefficient."

According to Dick Klingens, in the Netherlands the equation is usually written as y = ax + b or px + q or mx + n. He writes that the Dutch word for slope is "richtingscoëfficiënt", which literally means "direction coefficient."

In Austria k is used for the slope, and d for the y-intercept.

According to Julio González Cabillón, in Uruguay the equation is usually written as y = ax + b or y = mx + n, and slope is called "pendiente," "coeficiente angular," or "parametro de

direccion."

According to George Zeliger, "in Russian textbooks the equation was frequently written as y = kx + b, especially when plotting was involved. Since in Russian the slope is called 'the angle coefficient' and the word coefficient is spelled with k in the Cyrillic alphabet, usually nobody questioned the use of k. The use of b is less clear."

So why do we use m? Mostly habit... after all, you gotta' use something. I would hope some of my international readers would tell us what THEY use for the linear coefficient symbol in this slope intercept form of the equation; especially if they actually do use m.

Stay tuned.

ADDENDUM:

I got a couple of comments telling me that m is used for slope in Germany (y=mx+t) by Thony Christie, and one from Luke Robinson that in the UK they use y = mx+c.

Maria Miller added that, "In Finland, it is y = kx + b, and the term for 'k' is 'kulmakerroin' - literally 'angle multiplier'. "

And recently, Dave Marain, whose wonderful blog is appropriately titled Math Notations, sent the following:

" Pat, the best comment on my blog from years ago was from an Australian educator who said down under they spell it mlope"

An even older use in 1812 was in an American edition of Hutton's Course of Mathematics for the use of Academies. Hutton (Adrian) uses x=ny+c and y=mx+d in the same line, so his use seems more along the line of letting middle letters (k,l,m,n) stand for pronumerals (a word I derived from pronouns to indicate numbers that represented some single unspecified value) and x,y,z to represent true variable quantities. This is from Hutton's 5th and 6th edition and was "revised and corrected" by Robert Adrian. Adrian was an Irish-American considered one of the brightest mathematicians in the early 19th century. He independently found a method of least squares independent of Gauss (in fact published before Gauss' work was published).

It seems the use of middle letters in the alphabet for the linear coefficient of terms was pretty common near the beginning of the 19th century. For example, in factoring a trinomial the 1804The mathematical correspondent: containing, new elucidations, discoveries, and improvements, in various branches of the mathematics says "will always be the product of two rational factors (mx+p)(nx+q)."

And about four years after the first writing, a tweet fro Mathematical_A shared that, "Earliest known use of m for gradient in print is Vincenzo Riccati, 1757. He used y=mx+n.