“The curve was fit to the data points.”

“The curve was fitted to the data points.”

Which is correct? The answer can betray some strong opinions. “Using fit that way makes one sound like a backwoods rustic.” “Using fitted that way makes one sound like a pedant.” “Everyone knows that fitted is simply the adjectival form of fit.” In fact, the story is a bit more involved.

Fitted is indeed the adjectival form of fit (eg, fitted sheet), but it also can be used as the simple past or the past perfect verb form. However, whether fit is also acceptable when forming past tenses is in part a matter of geography. In British usage, fitted is considered always correct; however, many authorities of American usage—Merriam-Webster’s, for example—accept either fit or fitted,¹ and even among those holding that fitted is always correct, some—Bernstein, for example—make allowances for the use of fit in casual speech.² Such allowances often open the door for increased acceptance of a given usage in more formal contexts as well; indeed, whereas fitted was once invariably accepted for both the simple past and the past perfect, the -ed form is gradually going the way of the dodo.³ Moreover, when given the choice, many language users in the United States simply prefer fit over fitted.⁴

Such preferences aside, however, context can make a difference. For example, fitted is used when referring to the tailoring of clothing³; eg, “I was fitted for a new suit.” In other contexts, it can be helpful to consider if fit is being used in the sense of “to conform to a particular shape or size”¹; if so, some might hold that fit is correct for both the simple past and the past perfect.⁵ If, on the other hand, fit is being used in the sense of “to make or adjust to the right shape and size”¹—or the closely related meaning alluded to in the opening examples, “to adjust (a smooth curve of a specified type) to a given set of points”¹—fitted is used for the simple past and either fit or fitted for the past perfect.⁵

Fit to be tied? The bottom line:

●Referring to the tailoring of clothing? Use fitted for both the simple past and past perfect.

●Referring to something being a particular size or shape? If you’re British or Canadian, you’re safest using fitted for both the simple past and the past perfect; if you’re from the United States, you’ll be in good company using fit for both, but be aware that some authorities advocate the use of fitted in writing and formal speech.

●Referring to adjusting something (other than clothing) to a particular shape or size? Referring to the fitting of a curve to data? If you’re British or Canadian, you’re still safest using fitted in all instances. If you’re from the United States, you’re safest using fitted for the simple past and either fit or fitted for the past perfect—but again, using fit in the latter context might be frowned on.—Phil Sefton, ELS

1. Fit. Merriam-Webster’s Collegiate Dictionary. 11th ed. Springfield, MA: Merriam-Webster Inc; 2003:473.

2. Fit. In: Bernstein TM. The Careful Writer: A Modern Guide to English Usage. New York, NY: Athaneum; 1985:186.

3. O’Conner PT. Woe Is I: The Grammarphobe’s Guide to Better English in Plain English. New York, NY: Riverhead Books; 1996:65.

4. Fit vs. fitted—two options. Englishpage.com Web site. http://www.englishpage.com/irregularverbs/info/html. Accessed July 15, 2011.

5. Fitted vs fit. English-Test.net Web site. http://www.english-test.net/forum/ftopic19015.html. Accessed July 15, 2011.