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

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.

I almost forgot 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 or 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.

How can I 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.

What I can do if I have so much time to think these days?

What happens if I choose not to mention and let alone for each other instead?

I hardly have time to think these days.

I find the distinction that MacMillan makes between not to mention and the supposedly synonymous let alone and still (in decrepit

• decrepit) can be used as proof for a statement that lacks its meaning.
• 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.

There is a fallacy at the heart of this question.

Let’s say word W has two meanings W1 and W2.
Does a phrase have a meaning, is it merely a little different?
W1 and W2 are meanings in different situations. As others have said, meanings different can be used.

In India, word w also has three synonyms ( S1, S2, S3 ) listed in various dictionaries, thesauri, etc..

as it happens:

• S1 is a synonym for meaning W1 of word W
• S2 & S3 are synonyms for meaning W2 of word W

Is S2

• and S3 synonyms for S1, S2 & S3 in English?

Hence just because:

• not to mention and let alone are synonyms; and
• much less & still less are synonyms of let alone

it does not follow that

• much less & still less are synonyms of not to mention.

Why are some words on this blog synonyms etc.? I mean, merely explaining that, on the basis of the facts given in the question, it doesn’t necessarily follow that they are all synonyms of one another.

These are all constructions.
In layman’s terms, they have semantic properties, syntactic peculiarities, pragmatic
uses, metaphoric sources, and plenty of idiomaticity to go around.

Fortunately, let alone construction has been analyzed in great and precise detail in a famous paper by Fillmore, Kay, and O’Connor, “Regularity and Idiomaticity in Grammatical Constructions: The Case of Let Alone “, Language, Vol 64, Number 3 (1988:501-38).

Of course, this doesn’t explain the differences with the
other subjects, but it does give you an idea how complex the subject

can get.

I would like to point out that these are not really synonyms but represent a decreasing scale of likelihood or relevance:

I could not afford a bike, let alone a motorbike, much less a car and still less a limousine.

Is not in the same group, it is used slightly differently as explained in @Gnawme’s answer. For example,

the food was excellent, not to mention the wine!

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