We've dealt with the concept of euphony before, in our lessons on the French aspirated h and on liaisons. Euphony in French is the tendency to avoid having a word that ends in a vowel before a word that begins with a vowel. It's the reason why you have l'animal instead of le animal—it just "flows" better! In this lesson, we'll look at two specific instances of euphony, before the pronoun on and before the indefinite article un/une

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Take a look at the way on is used in this caption: 

Ce que l'on demande, c'est d'avoir uniquement

What we're asking is to have only

la photo de... de l'animal.

the photo of... of the animal.

Caption 17, Grand Lille TV - Des photos contre l'abandon des animaux

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You might be wondering what l’ is doing before on here. L’ is the contracted form of le and la (the), and on is a singular pronoun meaning "we," "they," or "one." But it doesn’t make any sense to say "the we." So what does the l’ mean here? Actually, it doesn’t really mean anything! In formal and written French, you’ll see l’on instead of on and l’un/l’une instead of un/une in certain situations for euphonic purposes.

There are two situations where l’on is preferred over on: 

1. After que (see the example above) and words that end in que, such as lorsque (when), puisque (since), and quoique (although). This is to avoid the contraction qu'on, which sounds the same as a rude French word that we won't mention here. 

2. After short words ending in a vowel sound, such as et (and), ou (or), où (where), and si (if):

Si l'on fait la queue, on... on a froid.

If we wait in line, we... we're cold.

Caption 11, Fanny parle des saisons - Activités

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And there are two situations where l’un/l’une is preferred over un/une:

1. When un/une is followed by a preposition (usually de or des):

Voici Indira, sans doute l'un des animaux de compagnie les plus insolites qui puissent exister.

Here is Indira, undoubtedly one of the most unusual pets that could possibly exist.

Caption 3, Angers 7 - Un lama en plein appartement

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2. At the beginning of a clause:

L'une des icônes principales de l'église est le martyr saint Mina.

One of the church's principal icons is the martyr Saint Mina.

Caption 15, LCM - Joyeux Noël... orthodoxe!

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As we mentioned, l’on and l’un/l’une are mainly used in formal and written French. In casual spoken French, you’ll often just see the words without the l’:

Ça fait longtemps qu'on attend ça, hein.

We've been waiting a long time for this, you know.

Caption 18, Alsace 20 - Rammstein à Strasbourg

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But since it’s always good to know the "proper" way of speaking, keep these rules in mind!