etymology - Not to Mention ≈ [Let Alone ≈ Much Less ≈ Still Less]

For brevity, I symbolise synonymity with ≈.  So  X ≈ Y  means  X and Y are synonyms.

From http://www.thefreedictionary.com/let+alone: let alone ≈ not to mention

From Merriam Webster: let alone ≈ much less ≈ still less.

Are these all synonyms? What are the similarities and differences between each?

Answer

MacMillan and I agree in disagreeing that not to mention and let alone are synonymous:

not to mention: used for adding a comment that emphasizes the main idea of what you have already said: The weather here is gorgeous, not to mention the wonderful food.

let alone: used for saying that something is even less likely to happen than another unlikely thing: I hardly have time to think these days, let alone relax.

still/much/even less: used after a negative statement in order to emphasize that it applies even more to what you say next: I am no one's spokesman, much less his.

You could make the case that let alone and much less are synonymous, since you could substitute one for the other:

I am no one's spokesman, let alone his.

I hardly have time to think these days, much less relax.

However, substituting not to mention and let alone for each other would result in sentences that sound contradictory:

The weather here is gorgeous, let alone the wonderful food.

I hardly have time to think these days, not to mention relax.

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I find the distinction that MacMillan makes between not to mention and the supposedly synonymous let alone and still/much/even less useful:

• The phrases let alone and still/much/even less reinforce a negative or unlikely statement that precedes them.


• The still/much/even less constructs reinforce the negativity of the preceding phrase by subtraction -- Negative statement, still/much/even less.


• In a similar manner, let alone reinforces the unlikeliness of the preceding phrase with an even more unlikely scenario -- I couldn't get a date with a reanimated corpse, let alone the prom queen.

Some grammarians insist that not to mention and let alone are synonymous; while you can find some cases where substituting one for the other might make sense, I generally find their meaning and usage distinct from each other.