In practice, which would be correct: "Does the word begin with a capital m?" or "Does the word begin with a capital M"?
In that situation I'm minded to use M, because that's what I'm referring to, but to ask " Does the word begin with an uppercase or lowercase _?" is not so obvious.
The OP asks whether the upper or the lower case version of a letter is the 'primary' form, thus making the other version 'subsidiary'. Should it be -
'M' - is a capital 'm'.
or
'M' - is a capital 'M'.
Similarly, should it be that -
'm' - is a lowercase 'M'.
or
'm' - is a lowercase - 'm'.
I think the answer is that it does not matter BUT that your usage should be consistent.
Apparently. there is some controversy over this. It seems that it's a matter of opinion if you should use capital M or capital m.
Of course, you can always rephrase the question to avoid the debate:
Does the word begin with a capital letter?
Is the first letter of the word capitalized?
Does the word begin with M or m?
Personally, when I refer to letters as letters, I put them in italics. (Some people will put them in quotation marks.) Also, if I'm making a specific distinction between the uppercase version and the lowercase version, especially when writing them next to each other, I use the version I want to draw attention to. (As in the third example sentence above.) If the case of the letter is unimportant to the discusion, I use the lowercase version in italics—which is what The Chicago Manual of Style recommends.
Depending on how you view it, "a capital M" is fine, because the phrase is describing what the thing already is—it would be analogous to saying "a yellow banana" or "a cold ice cube." It could perhaps be considered redundant, but redundancies are frequently used in English to make ideas explicit.
On the other hand, it could be considered illogical to say "a capital M" because of an implication that you are taking an M and turning it into a capital form. (But, of course, that doesn't exist.)
So, avoid this particular issue altogether and rephrase your sentence in a similar fashion to the example sentences I provided—or simply stick to whatever style you want to use, assuming it doesn't conflict with a style guide you need to adhere to.
REVISED
Should “mum/mom” be written with a capital ‘m’ or not?
I would suggest that letter mentioned should be left in lowercase, as a “capital M” would suggest there might be an uppercase version of M.
However, what do I know? Journalists writing for The New York Times use the uppercase
Still in high school, he has grown up with the notion that college is a word that should always be spelled with a capital “C.” source
Then again, the Mets are developing a bumper crop of players whose last names begin with a lowercase D.
In d’Arnaud’s case, the Mets simply flipped an uppercase “P” upside down. source
In some ways, uppercase “Internet” was always a bit of an anomaly, since it is not really a proper noun comparable to a company name or an official place name. The term internet (short for internetwork) described any linked network of computers, so the capital “I” served to distinguish the global network from other internets — a pointless distinction now, since “internet” is rarely used anymore in the generic sense. source
If the letter is at the beginning of a word, paragraph or chapter, the author or publisher might want to set it off differently from the rest of the text, this is known as the initial or drop capital.
