Fewer versus less is a debate in English grammar about the appropriate use of these two determiners. Linguistic prescriptivists usually say that fewer and not less should be used with countable nouns (e.g. apples or cars), and that less should be used only with uncountable nouns (e.g. water or happiness).
This distinction was first tentatively suggested by the grammarian Robert Baker in 1770, and it was eventually presented as a rule by many grammarians since then.
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The Daily Omnivore 