Q: I have done some googling, but I am still unsure of the difference, if any, between the words “proportional” and “proportionate.” My gut tells me you will respond that, outside of a mathematics discussion, the words are interchangeable.
A: Your gut is right. Aside from a few specialized meanings, these adjectives are so alike that any differentiation between them would be hair-splitting. They both mean in proportion.
“Proportional” and “proportionate” come from corresponding Latin adjectives, proportionalis and proportionatus. The chief difference is the suffixes, “-al” (-alis) and “-ate” (-atus), which can be used to form adjectives from nouns.
The natural question is, Why do we—and why did the Romans—need two such words?
As it happens, the ancient Romans had only one, the classical Latin proportionalis, which is the source of “proportional,” according to the Oxford English Dictionary.
English acquired the other word, “proportionate,” from the late Latin adjective proportionatus (proportioned, corresponding), which the OED dates from around 1250.
It seems likely that “proportionate” never would have entered English if medieval Latin scholars hadn’t invented proportionatus.
Interestingly, the English adjectives first appeared in writing at the same time—around 1397—and in the same work.
The OED’s earliest citations for both words are from John Trevisa’s Middle English translation of De Proprietatibus Rerum (“On the Properties of Things”), an encyclopedia written in Latin in the mid-1200s by a Franciscan monk, Bartholomaeus Anglicus.
Here’s “proportional,” from a section on the grafting of trees: “Among all graffynge of trees, the best is whan the graffe and the stok beth yliche … other of trees that haueth humour proporcional and acordynge either to other.”
And here’s “proportionate,” from a section about fingernails: “The nailes growen in lengthe & brede in quantite proporcionat to the fyngres.” (We’ve replaced the runic letter thorn with “th” throughout.)
The OED defines “proportional” here as meaning “that is in proportion, or in due proportion; related proportionately to something; corresponding, esp. in degree or amount.”
And it defines “proportionate” as meaning “proportioned, adjusted in proportion; that is in (due) proportion, proportional (to); appropriate in respect of quantity, extent, degree, etc.”
Apart from some specialized meanings, the definitions haven’t changed much since the 14th century. The words are as similar today as they were then—in the OED and in standard dictionaries.
For instance, both The American Heritage Dictionary of the English Language (5th ed.) and Merriam-Webster’s Collegiate Dictionary (11th ed.) define “proportionate” as meaning “proportional.” American Heritages adds “being in due proportion.”
The two dictionaries give “proportional” these meanings (we’ll paraphrase): (1) being in proportion; (2) corresponding or properly related in size, degree, etc.; (3) having the same or a constant ratio.
So which is the better one to use? That’s up to you. Use the one that sounds better to you in context. That’s what people have been doing for 600 years.
The same is true for the negative versions—“disproportional” versus “disproportionate,” which mean out of proportion. Although “disproportionate” is vastly more popular than “disproportional,” they’re equally legitimate and virtually interchangeable.
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