The difference between ‘less’ and ‘fewer’
The Economist explains
Many people insist on a bright-line distinction between “fewer” and “less”, and get quite agitated by the subject. David Foster Wallace’s novel “Infinite Jest” featured the Militant Grammarians of Massachusetts, who boycott stores with signs reading “12 items or less”. A few vigilantes have defaced such signs in real life. What is the distinction, and why does it matter?
Nouns can be “count nouns” and “mass nouns”. Count nouns are usually distinct things that can be counted, and take a plural: think “houses” or “shirts”. Mass nouns can’t usually be counted or made plural: think “water” or “oatmeal”. (They can sometimes be counted, as in a fancy restaurant offering several different waters, but “water” in ordinary use is otherwise a mass noun). In the traditional rule, “fewer” goes with count nouns and “less” with mass nouns. So it is “My sister has fewer shirts than I do”, but “My brother has less oatmeal than I do”. The rule was first proposed in this form in 1770 by Robert Baker in “Reflections on the English Language”.
But Baker expressed this as a preference, not a rule, perhaps because there are many shadings on it. The mass-count distinction does not always line up with the real-life properties of things: “clothing” is a mass noun (so it’s “less clothing”) but “clothes” is a count noun (so “fewer clothes”). Clothes are discrete items—like a typical count noun. And yet you can’t count them: “he is wearing four clothes” makes no sense. Meanwhile, some count nouns don’t represent discrete things at all. Take time and distance: years and miles are count nouns, but they represent arbitrary sections on a continuum. This probably is why many people find “I’ve lived here less than three years” more natural than “I’ve lived here fewer than three years”. And “less” is almost always more natural than “fewer” after one, in sentences like “that’s one less thing to deal with”.
Finally, there is register or style. “Fewer” is never used with mass nouns, but in casual speech, “less” is often used with count nouns. “She won’t go out with anyone with less than three cars” is fine for the barstool, but using this phrasing in print is likely to attract an editorial correction. The so-called rule has never reflected reality: as far back as the 9th century we find Alfred the Great writing swa mid laes worda, swa mid ma (“be it with less words or with more”). But it is a good guideline for formal writing—and good for keeping the Militant Grammarians of Massachusetts out of your supermarket.