Case when there is only one s
When there is only one s in the word, it can give either the sound [z] or the sound [s].
Indeed, the general rule is that the sound [z] occurs when the -s- is preceded and followed by a vowel.
Otherwise, it is the sound [s].
- une maison ==> two vowels before and after so sound [z]
- un magasin ==> two vowels before and after so sound [z]
- une veste ==> no vowel before and after so sound [s]
- elle danse ==> no vowel before and after so sound [s]
un oiseau (sound [z])
However, there are exceptions to the general rule.
In some compound words, even if the -s- is surrounded by vowels, it is still pronounced [s].
- un parasol
- un tournesol
- vraisemblable
- un contresens
- contresigner
There is also an exception in the other direction when there is a single -s- that is not surrounded by vowels and is pronounced [z] : du vinaigre balsamique.
Case when there are 2 ss
When two -ss- follow each other, they always form the sound [s], even if they are surrounded by vowels.
- un dessert ==> sound [s]
- un coussin ==> sound [s]
- une classe ==> sound [s]
- When there is 1 s followed and preceded by a vowel, it forms the sound [z] (e.g. un désert) except in words that have been composed and which are exceptions (e.g.: vraisemblablement).
- When there are 2 ss, there is always sound [s] (e.g.: un dessin)
Exercises
Practice this rule with the following exercises : Exo 1 - 2 - 3