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The unchanging 'demi' prefix before a noun

French uses the word demi to express the concept of a half. While this seems straightforward, English speakers often trip over how it changes, or rather how it does not change, depending on where it sits in a sentence.

We see this constantly in everyday vocabulary like demi-heure and demi-sœur. The spelling of demi shifts based on a very strict rule about its position relative to the noun.

Word list
demimA1 half demi-heure, demi-sœur demi-heurefA2 half an hour une demi-heure faire demi-tourB1 to turn around faire demi-tour demi-cerclemB2 semicircle un demi-cercle, des demi-cercles (demi- stays unchanged) demi-finalefB2 semifinal une demi-finale demi-frèremB2 half brother un demi-frère demi-sœurfB2 half sister une demi-sœur
Before the noun

When placed before a noun and connected with a hyphen, the prefix demi- acts as an unchanging block. This means it never changes to match the gender of the noun it goes with.

Even if the noun is feminine, like heure, sœur, or finale, you must write demi- without an extra e.

Le train arrive dans une demi-heure.
(The train arrives in half an hour.)
Ma demi-sœur habite à Paris.
(My half sister lives in Paris.)
Son demi-frère est très grand.
(His half brother is very tall.)
Nous allons regarder la demi-finale ce soir.
(We are going to watch the semifinal tonight.)
Plurals with the prefix

What happens when you need to talk about multiple half-hours or several half brothers? The rule remains firm. The prefix demi- stays exactly the same.

You only add an s to the main noun at the end of the word.

J'ai attendu pendant trois demi-heures.
(I waited for three half-hours.)
Il a invité ses deux demi-sœurs au mariage.
(He invited his two half sisters to the wedding.)
After the noun

The word demi does change when it comes after the noun. When you want to say "and a half", you use the phrase et demi. In this position, it acts like an adjective and must agree in gender with the noun.

If the noun is feminine, you add an e to make it et demie. If the noun is masculine, it stays et demi.

Le match a duré deux heures et demie.
(The match lasted two and a half hours.)
J'ai habité au Canada pendant un an et demi.
(I lived in Canada for a year and a half.)

However, you must watch out for a major exception. Even though it agrees in gender after the noun, it never changes for singular or plural. It will never take an s, even if the noun is plural. You will write deux heures et demie, but never write demies.

Summary

As a rule of thumb, remember that before a noun with a hyphen, demi- never changes. When placed after a noun, it adds an e for feminine nouns, but it never takes an s to become plural.