Une demi-baguette ou une moitié d'une?

Question

Like many other commenters, I am confused by the the expected answer to this question: "Write 'He ate half a baguette'."

The phrasing is, at best, ambiguous in English. Either 1) he had a half baguette and he ate the whole thing, or 2) he had a whole baguette and ate half of it. Either one is a possible interpretation, and without more context and with this exact English phrase, I would assume case 2.

If indeed case 2 is intended (he ate one half of the whole baguette that he had), could one not say in French, "Il a mangé une moitié d'une baguette."? The lesson has these examples that seem like the same situation and use moitié : J'ai bu la moitié de la bouteille and Il ne mange qu'une moitié du biscuit.

The lesson also says: "Depending on context, Un/une demi-[nom] can either refer to half a full-sized item (i.e. bucket) or to one of these half-sized items . . . but you use demi- in both cases." That seems to say that demi-baguette would be correct even if he ate half of the full-sized one he had, but I don't understand why une moitié would not also work. What is the difference between the baguette and the bouteille ("J'ai bu la moitié de la bouteille ")?

Merci!

Céline Pickard · Accepted Answer

Bonjour N.,
Une demi-baguette means "half a baguette" as an amount/type (i.e. half a baguette's worth, with no particular baguette in mind). In "He ate half a baguette", you're describing how much he ate, so this is the natural choice.
La moitié de la baguette means "half of the baguette". This points to a specific baguette already in view, of which he ate one half. The definite article "la" signals a particular whole.
This is exactly why the lesson's examples use moitié: la moitié de la bouteille, une moitié du biscuit ... Both refer to a specific item (la bouteille, le biscuit). The difference from the baguette sentence isn't the noun. It's that there's no specific baguette here: it's just "half a baguette" as a quantity. So, demi- fits.
As for une moitié d'une baguette, it's not idiomatic because moitié takes a definite complement (la moitié de la baguette, not une moitié d'une baguette). So, if you wanted the "half of a specific baguette" meaning, it would be il a mangé la moitié de la baguette.
Take a look here too:
"Should not [La moitié de baguette] work as well as [Une demi-baguette]?
I hope this is helpful.
Bonne journée !

Nathan Rochet · Answer

Bonjour Céline,This answer is helpful with regard to "half a baguette".  If I understand correctly, half a baguette is a measurement of baguette and thus always a demi-baguette and never une moitié d'une baguette.  "Moité" is only used if a specific baguette is under discussion (for example, he ate half of the baguette that I was saving for my lunch!) . Is the use of demi-baguette as a measurement special and idiomatic, or does the same rule apply in other situations where an amount or quantity can be measured as half a non-specific item?She ran a half-marathon then ate half a banana.He has a reduced appetite and can usually only eat half a meal.I often eat half a candy bar after school.On average, the builders can build about half a house per month.Each deviled egg is made with half a spoonful of mayonnaise and half a hard-boiled egg.I can't believe he ate half a pizza and then half a cake!Merci!

Céline Pickard · Answer

Bonjour N.,
Using "une demi-baguette" is not idiomatic, it is a general rule when using "demi-..." (i.e.  not something special to baguettes).
So, remember:

demi- + [noun] = “half a [...]” as an amount/quantity, with no specific whole in view.

la moitié de + [specific noun] = “half of [a particular, identified thing].” Needs an article (la moitié de la baguette / de ce gâteau / de mon repas).

Attention: 
"half-marathon" is a fixed expression and uses semi- (not demi-) -> un semi-marathon
 
I hope this is helpful.
Bonne journée !