According to prescriptive grammar, "fewer" should be used (instead of "less") with nouns for countable objects and concepts (discretely quantifiable nouns or count nouns). According to this rule, "less" should be used only with a grammatically singular noun (including mass nouns) and only when they suggest "a combination into a unit, a group, or an aggregation: less than $50 (a sum of money); less than three miles (a unit of distance)". However, descriptive grammarians (who describe language as actually used) point out that this rule does not correctly describe the most common usage of today or the past and in fact arose as an incorrect generalization of a personal preference expressed by a grammarian in 1770.
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This rule can be seen in the examples "there is less flour in this canister" and "there are fewer cups (grains, pounds, bags, etc.) of flour in this canister", which are based on the reasoning that flour is uncountable whereas the unit used to measure the flour (cup etc.) is countable. Nevertheless, even most prescriptivists accept the most common usage "there are less cups of flour in this canister" and prescribe the rule addition that "less" should be used with units of measurement (other examples: "less than 10 pounds/dollars"). Prescriptivists would however consider only "fewer cups of coffee" to be correct in a sentence such as "there are fewer cups of coffee on the table now", where the cups are countable separate objects, although most people now and in the past use "less" even in such cases. In addition, "less" is recommended in front of counting nouns that denote distance, amount, or time. For example, "we go on holiday in fewer than four weeks" and "he can run the 100 m in fewer than ten seconds" are not advised.
Some prescriptivists argue that even the extremely rare and completely unidiomatic "one fewer" should be used instead of "one less" (both when used alone or together with a singular, discretely quantifiable noun as in "there is one fewer cup on this table"), but Merriam–Webster's Dictionary of English Usage clearly states that common usage dictates "one less" in all cases.
Current usage
The comparative less is used with both count and uncount nouns in some informal discourse environments and in most dialects of English. In other informal discourse however, the use of fewer could be considered natural. Many supermarket checkout line signs, for instance, will read "10 items or less"; others, however, will use fewer in an attempt to conform to what is incorrectly perceived as required by the prescription although this is in fact a clear case of hypercorrection as explained in Pocket Fowler’s Modern English Usage. A British supermarket chain replaced its "10 items or less" notices at checkouts with "up to 10 items" to avoid the issue. It has also been noted that it is less common to favour "At fewest ten items" over "At least ten items" – a potential inconsistency in the "rule", and a study of online usage seems to suggest that the distinction may, in fact, be semantic rather than grammatical. Likewise, it would be very unusual to hear the unidiomatic "I have seen that film at fewest ten times."
Historical usage
Less has always been used in English with counting nouns. Indeed, the application of the distinction between less and fewer as a rule is a phenomenon originating in the 18th century. On this, Merriam–Webster's Dictionary of English Usage notes:
As far as we have been able to discover, the received rule originated in 1770 as a comment on 'less': This Word is most commonly used in speaking of a Number; where I should think Fewer would do better. "No Fewer than a Hundred" appears to me, not only more elegant than "No less than a Hundred," but more strictly proper. (Baker 1770). Baker's remarks about 'fewer' express clearly and modestly – 'I should think,' 'appears to me' – his own taste and preference....Notice how Baker's preference has been generalized and elevated to an absolute status and his notice of contrary usage has been omitted."The oldest use that the Oxford English Dictionary gives for less with a count noun is a quotation from 888 by Alfred the Great.
Swa mid læs worda swa mid ma, swæðer we hit yereccan mayon. With less words or with more, whether we may prove it.This is in fact an Old English partitive construction using the "quasi-substantive" adverb læs and the genitive worda ("less of words"). When the genitive plural ceased to exist, less of words became less words, and this construction has been used since then until the present.
The Cambridge Guide to English Usage notes that the "pressure to substitute fewer for less seems to have developed out of all proportion to the ambiguity it may provide in noun phrases like less promising results". It describes conformance with this pressure as a shibboleth and the choice "between the more formal fewer and the more spontaneous less" as a stylistic choice.