Demi, moitié, etc = Half

Hi Graham, I don't think you can slice it that way. In my understanding it is perfectly OK to say, "Donnez-moi cette demi-baguette."

The distinction is with respect to using it as an adjective (demi/e) or a noun (la moitié).

Une demi-page est rouge. -- A half-page is red.
Une moitié de la page est rouge. -- One half of a page is red.

There are foten used colloquialisms and appreviated forms evolving around demi.

Il est six heures et demie. -- It is half past six.
This is an abbreviation of "...six heures ete une demi-heure".

On se renconte à demie. -- We are meeting at half-past. (This assumes an agreement about which hour one is talking).

Je voudrais un demi. -- I would like a half-pint of beer. (This works only colloquially  in a bar when ordering beer.)

-- Chris (not a native speaker).

Darn, again the typos....:

foten --> often.
appreviated --> abbreviated
ete --> et

-- Chris.

Hi Dragana,

the rule is this: when "demi" comes before the noun, it is always and invariable "demi". When it succeeds the noun, it changes according to the gender.

Une demi-heure. -- Half an hour.
Une heure et demie. -- One and a half hour.

-- Chris (not a native speaker).

I answered this question in your other post. -- Chris.

Bonjour Nick,
Let's see if this helps any.
Toute la journée, il n'a mangé que la moitié d’un biscuit
All day, he ate only one-half of a biscuit
compare the above to this phrase:
Il n'a mangé que la moitié du biscuit
He ate only half of the biscuit
In other words, the differentiation comes whether is un biscuit, a biscuit, or du biscuit, remember that «du» is the shortened form of «de le» hence the biscuit.
J'espère que cela vous aidera.
Bonne chance,
Ron

Bonjour Sue !

Here, because it's a whole composed of only two halves, you will use "la moitié de" as such:
"Il a mangé la moitié des pommes."

That means there was a specific quantity of apples, and he ate half of this quantity.

I hope that's helpful!
À bientôt !

Bonjour Tatiana !

Here there's a nuance of meaning between une demi-baguette and la moitié de [la] baguette.

Une demi-baguette means "one half of baguette" or "half a baguette": here it's a general characteristic of the object, as opposed to a full baguette.

La moitié de [la] baguette means "the half of baguette" or "half the baguette", implying that we're talking about a specific baguette here, of which you'd want half.

The sentence here could be heard in a boulangerie, where you would ask for half a baguette, hence une demi-baguette.

I hope that's helpful!

À bientôt !

Bonjour Diana !

In most cases meaning "half", "demi(e)" and "une/la moitié de" are equivalent.
Une demie-baguette / la moitié d'une baguette

However, "demi(e)" refers more to a characteristic of the object (it's a half-thing) rather than a quantity as such.
So in the case of chocolate, you only say "un demi-chocolat" if you actually ate a half (piece of) chocolate, but if you refer to half a given quantity of chocolate, then it will be "la moitié du chocolat" :)

I hope that's helpful!
À bientôt !

Bonjour Chris !

Une moitié de la bouteille means "one half of the bottle", whereas la moitié de la bouteille means "half the bottle".

I hope that's helpful!
À bientôt !

Bonjour John,

Now that's an interesting topic!

Here's what I can say for sure: when you consider a half bottle, as in half full, you will use "une demie bouteille".
Now if you're talking about those smaller bottles you can buy in shops, we don't really have a specific word for them, other than "une petite bouteille de vin" or again "une demie bouteille de vin".
You can refer to "un demi" when you order what is equivalent to half a pint [more for beer though] or about 25cl / a quarter of a litre.

Voilà ! I hope that answers your question!