What's the difference between un and l'un?

Un is a determiner and needs a noun after it: un livre. L'un is a pronoun and stands alone: l'un de ces livres. The l' carries no meaning of its own and does not make the phrase definite.

Is it un de mes amis or l'un de mes amis?

Both are correct and mean the same thing. L'un de is slightly more formal and is generally preferred when the phrase opens the sentence: l'un de mes amis m'a raconté cette histoire.

Why do you have to say l'un et l'autre?

Because the pairing with l'autre is fixed. French requires the two halves to match in shape, so a bare un cannot appear opposite l'autre in any of these expressions.

How do you say each other in French?

The pronominal verb usually suffices — ils s'écrivent means they write to each other — but when the meaning could be reflexive instead, add l'un l'autre: ils se regardent l'un l'autre. For more than two, use les uns les autres.

Where does the preposition go in l'un de l'autre?

Between the two halves, matching whatever the verb requires: parler à gives l'un à l'autre, avoir besoin de gives l'un de l'autre, être fait pour gives l'un pour l'autre. It is never placed in front of the pair.

French grammar · Intermediate

Un vs. L'un in French: One, A, and One of Them

Written by Camille Laurent, Lead French Curriculum Editor · Reviewed May 2026

Un is the article a and the number one; it goes in front of a noun. L'un is a pronoun meaning one of them, and it stands on its own. The l' is not a definite article doing any work — it is a fossil that survives because French dislikes starting a phrase with a bare un, and it becomes compulsory as soon as l'autre is in the sentence.

L'un de mes frères vit à Lyon, l'autre à Nantes.

One of my brothers lives in Lyon, the other in Nantes.

What it is

The difference is grammatical rather than semantic. Un is a determiner and needs a noun to attach to. L'un is a pronoun and replaces the noun entirely, which is why it turns up in front of de, before a verb, and in every pairing with l'autre.

J'ai un frère. L'un de mes frères est médecin. — I have a brother. One of my brothers is a doctor.

How to spot it

Check whether a noun follows immediately. If it does, you want un. If the next word is de, ou, et, ni, or a verb, you want l'un.

un livre intéressant — an interesting book (article + noun)
l'un de ces livres — one of these books (pronoun)
l'un ou l'autre — one or the other
ni l'un ni l'autre — neither one nor the other

L'une is the feminine, les uns and les unes the plurals. All four keep the l'.

Un vs. L'un Quick Reference

Which form, and why

Structure	Form	Example
article before a noun	un / une	un chien, une maison
the number	un / une	j'en veux un
one of a group	l'un / l'une (or un / une)	l'un de mes amis
paired with l'autre	l'un (obligatory)	l'un et l'autre
in a negative pair	ni l'un ni l'autre	ni l'un ni l'autre n'est venu
a choice between two	l'un ou l'autre	prends l'un ou l'autre
sequence	l'un après l'autre	ils sont entrés l'un après l'autre
reciprocity	l'un l'autre / les uns les autres	ils s'aident les uns les autres

Common Un vs. L'un Examples in French

Each form in the position that requires it:

Un as an Article or a Number

J'ai acheté un parapluie.: I bought an umbrella.
Il ne reste qu'une place.: There's only one seat left.
Une seule personne a répondu.: Only one person answered.
Donne-m'en un.: Give me one of them.

As a number it can stand alone after en, but that is the pronoun un, not l'un.

L'un Picking Out a Member

L'un de mes collègues part à la retraite.: One of my colleagues is retiring.
L'une des portes était ouverte.: One of the doors was open.
C'est l'un des meilleurs films de l'année.: It's one of the best films of the year.
L'un d'entre eux a tout avoué.: One of them confessed to everything.

Un de mes collègues is also correct. L'un de is the more careful form, and it is preferred at the start of a sentence.

Paired with L'autre

L'un et l'autre me plaisent.: I like both of them.
Prends l'un ou l'autre.: Take either one.
Ni l'un ni l'autre n'a téléphoné.: Neither of them called.
Ils sont entrés l'un après l'autre.: They came in one after the other.

Once l'autre appears, l'un is obligatory. Un et l'autre is not an option in French.

Les Uns and Les Autres

Les uns disent oui, les autres non.: Some say yes, others say no.
Aidez-vous les uns les autres.: Help one another.
Ils se méfient les uns des autres.: They're wary of one another.
Les unes sont rouges, les autres bleues.: Some are red, the others blue.

The plural forms are used whenever more than two people or things are involved.

Why the L' Is There at All

It is not a definite article in any meaningful sense, and knowing that explains where it appears and where it does not.

Un Needs a Noun, L'un Does Not

Un is a determiner: its job is to introduce a noun. L'un is a pronoun: it stands in for one. That single distinction predicts every position each of them can occupy.

Un ami m'a prévenu, et l'un de ses collègues aussi.

A friend warned me, and so did one of his colleagues.

Noun after it → un. Nothing after it → l'un.

L' Is a Support, Not a Meaning

The l' comes from Old French and survives because a lone un at the head of a phrase sounded weak. It adds no definiteness, which is why l'un de mes amis and un de mes amis mean exactly the same thing.

L'un de nous devra rester.

One of us will have to stay.

l' is euphonic, not definite.

L'autre Makes It Obligatory

In any pairing with l'autre, the first element has to be l'un. This is fixed and admits no variation, in the affirmative, the negative, and the sequential forms alike.

Elles se ressemblent, l'une comme l'autre.

They look alike, both of them.

l'un et / ou / ni / après l'autre.

It Agrees in Gender and Number

L'un has four forms: l'un, l'une, les uns, les unes. They agree with whatever they refer back to, and l'autre agrees alongside them.

L'une des deux sœurs est partie, l'autre est restée.

One of the two sisters left, the other stayed.

l'un, l'une, les uns, les unes.

Prepositions Slot Between the Two Halves

When the reciprocal relationship involves a preposition, that preposition goes between l'un and l'autre rather than in front of the pair.

Ils ont confiance l'un en l'autre.

They trust each other.

l'un de l'autre, l'un à l'autre, l'un pour l'autre.

Common Mistakes with Un vs. L'un

Incorrect: Un et l'autre sont possibles. — Both are possible.

Correct: L'un et l'autre sont possibles. — Both are possible.

As soon as l'autre appears, the first half has to be l'un. The pairing is fixed and does not allow a bare un.

Incorrect: Ils s'aident un l'autre. — They help each other.

Correct: Ils s'aident l'un l'autre. — They help each other.

The reciprocal phrase is l'un l'autre in full. Dropping the l' from the first half leaves an expression that does not exist.

Incorrect: Ils ont besoin de l'un l'autre. — They need each other.

Correct: Ils ont besoin l'un de l'autre. — They need each other.

The preposition belongs between the two halves, not in front of them. Avoir besoin de gives l'un de l'autre, and parler à gives l'un à l'autre.

Incorrect: L'un homme est venu. — A man came.

Correct: Un homme est venu. — A man came.

L'un is a pronoun and cannot be followed by a noun. Once a noun is present, only the article un is possible.

L'un l'autre and Reciprocal Verbs

The main working use of l'un is to make reciprocity explicit when a pronominal verb on its own would be ambiguous.

Removing the ambiguity

Ils se regardent can mean they look at themselves or they look at each other. Adding l'un l'autre forces the reciprocal reading.

Ils se regardent l'un l'autre.: They're looking at each other.
Elles s'encouragent l'une l'autre.: They encourage each other.
Nous nous soutenons les uns les autres.: We support one another.

For the reflexive reading instead, add soi-même or eux-mêmes: ils se regardent eux-mêmes dans le miroir.

When the verb has a preposition

If the verb takes à or de, that preposition reappears between the two halves. The pronominal pronoun stays in place as well.

Ils parlent l'un à l'autre.: They talk to each other.
Elles ont besoin l'une de l'autre.: They need each other.
Ils se moquent l'un de l'autre.: They make fun of each other.
Ils sont faits l'un pour l'autre.: They're made for each other.

The preposition is not repeated in front of l'un: never à l'un à l'autre.

Fixed expressions

A few phrases built on l'un have set meanings that cannot be worked out from the parts.

L'un dans l'autre, ça s'est bien passé.: All in all, it went well.
C'est tout l'un ou tout l'autre avec lui.: It's all or nothing with him.
Les uns et les autres ont leur part de responsabilité.: Everyone involved bears some responsibility.

L'un dans l'autre is the everyday way to say on balance, and it has nothing to do with anything being inside anything.

Un vs. L'un FAQs

What's the difference between un and l'un?: Un is a determiner and needs a noun after it: un livre. L'un is a pronoun and stands alone: l'un de ces livres. The l' carries no meaning of its own and does not make the phrase definite.
Is it un de mes amis or l'un de mes amis?: Both are correct and mean the same thing. L'un de is slightly more formal and is generally preferred when the phrase opens the sentence: l'un de mes amis m'a raconté cette histoire.
Why do you have to say l'un et l'autre?: Because the pairing with l'autre is fixed. French requires the two halves to match in shape, so a bare un cannot appear opposite l'autre in any of these expressions.
How do you say each other in French?: The pronominal verb usually suffices — ils s'écrivent means they write to each other — but when the meaning could be reflexive instead, add l'un l'autre: ils se regardent l'un l'autre. For more than two, use les uns les autres.
Where does the preposition go in l'un de l'autre?: Between the two halves, matching whatever the verb requires: parler à gives l'un à l'autre, avoir besoin de gives l'un de l'autre, être fait pour gives l'un pour l'autre. It is never placed in front of the pair.