Compound adjectives are formed by joining two words with a hyphen. The rules for making them plural or feminine depend on the types of words being combined.
Invariable Word + Adjective
If the first part of a compound adjective is an adverb, a preposition, or an adjective acting as an adverb, it is invariable (it never changes). Only the second word agrees in gender and number with the noun it modifies.
For example, in nouveau-né (newborn), nouveau acts as an adverb meaning "newly." Therefore, only né changes to match feminine or plural nouns. The same rule applies to words starting with avant- (before), bien- (well), or mort- (as in mort-né).
| The midwife looks at the newborn girls. | La sage-femme regarde les filles nouveau-nées. |
| I participate in the pediatrician's penultimate meetings. | Je participe aux avant-dernières réunions du pédiatre. |
| We must avoid stillborn ideas. | Il faut éviter les idées mort-nées. |
| The beloved children eat a lot of it. | Les enfants bien-aimés en mangent beaucoup. |
Adjective + Adjective
If a compound adjective is made up of two adjectives, both parts must agree in gender and number with the noun they describe.
For example, in premier-né (firstborn) and sourd-muet (deaf-mute), both halves of the hyphenated word are adjectives. You must make both parts feminine or plural to match the noun.
| The firstborn girls are often premature. | Les filles premières-nées sont souvent prématurées. |
| The midwife speaks to the deaf-mute mothers. | La sage-femme parle aux mères sourdes-muettes. |