Like how the antonym of uppercase is lowercase, and of majuscule is minuscule, what is capital's own antonym?
Alternatively, if capital has no unique antonym of its own, how did it come to refer to uppercase letters without one?
Like how the antonym of uppercase is lowercase, and of majuscule is minuscule, what is capital's own antonym?
Alternatively, if capital has no unique antonym of its own, how did it come to refer to uppercase letters without one?
[It's never a good idea to ask two questions at once, but to answer the second question...]
The word capital came to have a meaning without a specific antonym because it ultimately derives from the Latin caput, "head":
II. Standing at the head.
5. Formerly of a letter or word: standing at the head of a page, passage, etc. Now of a letter: having the distinctive form and size used to begin a sentence, proper name, etc.— OED (may be paywalled)
The oldest citation is from the Wycliffe Bible of 1382.
If it were to have an antonym of similar derivation, it would be corporal, but that word has never had that particular meaning, according to OED. Presumably this is because capital never needed an antonym — there were letters of a particular style (uncial, blackletter...), and capital letters stood out on their own. It would be a mistake to hijack corporal and force it into a new meaning nowadays.
When I was at school, we used the term little letters, probably because it's nicely alliterative.
You can call the minuscule letters "small" letters. Here are a couple of YouTube videos that show the usage: