Why would an interrogative pronoun be in the genitive (other than meaning "whose")?

Question

I am doing some Rosetta Stone and the phrase is:

Cuius magnitudinis libo eges?

The meaning is apparently supposed to be "What size cake do you need?" There are other similar interrogatives in the genitive in the same exercise set. For example, "Cuius coloris capilli tui sunt? (What color is your hair?)

However, I don't understand the grammar of this. According to my reading of the grammar on relative pronouns at Dickinson (https://dcc.dickinson.edu/grammar/latin/relative-interrogative-and-indefinite-pronouns) the genitive is only used with interrogative pronouns when the meaning is "whose". I would expect the sentence to be using either the nominative, or in the egeo to use the ablative of separation, such as:

Qua magnitudine libo eges?

I cannot find in Dickinson the use of the genitive for an interrogative. What am I missing?

Answer 1

Cuius isn't an interrogative pronoun here, but an interrogative adjective (Allen & Greenough §148.b) modifying magnitudinis. The genitive is a genitive of description (or what A&G §345 calls 'genitive of quality'): 'You need a cake of what size?' (Likewise for cuius coloris capilli tui sunt: 'Your hair is of what color?')

You won't find a special section in your grammar about this use of the genitive of the interrogative specifically, because it's just a run-of-the-mill use of the genitive; it just happens to involve an interrogative in this instance.

The English translation given ('What size cake do you need'), though perfectly acceptable English, is misleading here, because 'size' is the object of 'need'; however, in the equivalent Latin sentence, what's needed is the cake, not the size.

Although the sentence could technically be phrased as qua magnitudine libo eges, it's important to note that qua magnitudine would be an ablative or description/ablative of quality (A&G §415), not the ablative of separation with eges: libo still fills that function.

Answer 2

To understand the grammar of the Latin question, you may first need to understand the grammar of the English question so you can leave it behind. The noun "size" has peculiar grammar here, roughly only about 100 years old, briefly explained in an English Language Learners answer here. Non-native speakers are often baffled by the lack of a preposition before the noun ("What size of cake do you need?") or the choice of a noun rather than an adjective ("How big a cake do you need?"), both of which are more consistent with the rest of English grammar and parallel Romance-language grammar.

The noun "color" has the same peculiarity in English. I once baffled my Italian teacher trying to render "What color is…" in Italian. The correct Italian is Di che colore è…—the equivalent of the genitive in Latin.

Another clue is to consider the equivalent indicative sentence, or a full answer in the first person:

Libo magno egeo.

Capilli mei sunt fusci.

To make these into questions, you need some sort of interrogative word in place of the bold adjectives and maybe move it to an emphatic place in the sentence. My command of Latin is not strong enough for me to be sure of the most idiomatic phrasings, but I would think this is most natural, using quantum as an interrogative adverb:

Quantum magno libo eges? ("How big a cake do you need?")

or even using quanto as an interrogative adjective:

Quanto libo eges? ("What size cake do you need?", but maybe "How much cake do you need?")

There is no abstract adjective corresponding to fusci, so to make a question, you'll need to switch to the noun color, and that will require the genitive case, because it still needs to modify capilli:

Cuius coloris sunt capilli tui?

So that's the answer to your question: to get a noun, that names what you are asking for, to modify another noun. (Strictly speaking, cuius is an interrogative adjective here rather than a pronoun.)

The same works for converting magno to a noun, of course:

Cuius magnitudinis libo eges?