Anyone who has ever spoken to a pedant or a linguist will be more than aware that less and fewer are to be confused with one another at a great risk. Used as comparatives, less is used for mass nouns (‘less chocolate’), while fewer for count nouns (‘fewer pieces of cake’).
A lot of people, however, use less when they mean fewer – though never the other way around; for some reason saying ‘less biscuits’ sounds better than saying ‘fewer cake’. On the flip side, more is used for both mass and count nouns, causing no such confusion there.
Where did this obscure rule come from?
All three words, less, fewer and more, come from Old English. Less was lǣs or lǣssa in Old English, fewer from Old English fēawe with the added suffix –er, and more from Old English māra. Which means we can’t use the reason of another word coming into the English language from, say, Latin, and this creating a need for two distinct meanings. In fact, in the times of King Alfred, the guy who stopped the Vikings from pillaging the whole country, less was used before plural nouns. Saying ‘I want less pancakes’ was just as correct as saying ‘I want less pancake mix’.
It was in 1770 (the good old Age of Reason cropping up in our etymology again…) that Baker decided that ‘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).
Just as Lowthe’s rule that a double negative makes a positive caught on, the overly self-conscious social climbers in the 18th century clung to this thought: if you followed it, you were going places. Otherwise, you were obviously a peasant. What we see today; the rule linguists lord over others (guilty as charged), simply comes from the whim of our friend Mr. Baker.