one less

We all know that we should use fewer when we are dealing with discrete units that are countable, and less when we are dealing with a noun that cannot be counted.  We say that there are fewer apples in the bowl now that we have eaten some of them, and that there is less butter now that we have spread some on our toast.  If we side with popular usage and say less apples we could well be sternly corrected.

But what happens when we specify that there is one less. Immediately we are in a world of counting so it really should be one fewer.  There is one fewer apple in the bowl. It sounds slightly better as There is one apple fewer in the bowl. But they both sound odd, whereas one less apple in the bowl sounds fine.

This is an instance where popular usage has won.  We label one less as an idiom and move on.  Saying that something is an idiom means that you don’t try to justify or explain it. It just is.  It is a handy strategy.