A compound noun is a lexicalized word formed from two elements or several elements that function together as a single noun. It often denotes a precise reality whose meaning is not always deducible from the simple sum of its parts. In the study of Noms, it must be recognized as a unit of meaning, spelling and agreement.

Compound nouns are frequently built by noun + noun, by adjective + noun, or by verb + noun. They can also contain a form related to a preposition, often with de or d', as in traditional compounds. The first element may retain a verbal value in forms of type verb + noun.

IdéeIdea	ExempleExample
Nom et nomNoun and noun	un porte monnaiea wallet
Adjectif et nomAdjective and noun	une grand mèrea grandmother
Verbe et nomVerb and noun	un casse noixa nutcracker
Forme avec deForm with de	un arc en ciela rainbow

The writing varies according to the structure of the compound noun and according to usage fixed by the dictionary. Some nouns take a hyphen, others are written with a space, and some historical forms still have variants. The orthographic reforms of 1990 and regional usages, notably in Canada and Belgium, can maintain different choices.

IdéeIdea	ExempleExample
Trait d’unionHyphen	un arc en ciela rainbow
EspaceSpace	une pomme de terrea potato
Variante historiqueHistorical variant	un chef d œuvrea masterpiece
Usage régionalRegional usage	un casse noixa nutcracker

Gender is generally determined by the head of the compound noun, often the last element. The determiner agrees with the entire compound noun in gender and number, as for other nouns studied in Gender Nominal and Nominal Number. The plural is often marked only on the last element, but some forms retain particular plurals or invariable forms.

IdéeIdea	ExempleExample
Noyau finalHead element	une grand mèrea grandmother
Pluriel sur le dernier élémentPlural on the last element	des porte monnaieswallets
Déterminant accordéDeterminer agreement	les arcs en cielthe rainbow arcs
Pluriel variableVariable plural	des chefs d œuvremasterpieces

A compound noun often has a lexicalized meaning, that is to say a global sense that goes beyond that of its components. The meaning of curfew is not understood literally as the combination of to cover and fire, because the expression denotes a specific measure. This semantic autonomy explains why one must consult the dictionary for spelling and for the exact usage.

IdéeIdea	ExempleExample
Sens idiomatiqueIdiomatic meaning	un couvre feua curfew
Sens fixéFixed meaning	un chef d œuvrea masterpiece
Sens globalGlobal meaning	une grand mèrea grandmother

Some forms change according to regions, the typographic tradition, or the evolution of the written language. Speech often simplifies written groupings, which can erase a hyphen in speech. For unstable forms, the safest standard remains the attestation in the dictionary.

RégionRegion	Mot ou expressionWord or expression	Définition régionaleRegional definition	ExempleExample
FranceFrance	Trait d’unionHyphen	L’écriture traditionnelle conserve souvent le trait d’union dans les noms composés fixés.The traditional writing often keeps the hyphen in fixed compound nouns.	En France, un arc en ciel reste une forme reconnue, si l’usage la confirme.In France, a rainbow remains a recognized form, if usage confirms it.
CanadaCanada	PlurielPlural	L’usage régional peut favoriser des pluriels ou des graphies différentes selon les institutions.Regional usage may favor plurals or different spellings depending on institutions.	Au Canada, des casse noix peut varier selon la norme retenue.In Canada, nutcrackers may vary according to the standard adopted.
BelgiqueBelgium	GraphieSpelling	La tradition belge peut conserver certaines écritures plus stables ou plus anciennes.Belgian tradition may preserve certain spellings that are more stable or older.	En Belgique, un chef d œuvre apparaît souvent selon l’usage éditorial.In Belgium, a masterpiece often appears depending on editorial usage.

Compound nouns are lexical units formed from several elements, whose spelling, gender and plural must be learned as a whole. Their structure can be transparent or idiomatic, with forms of noun+noun, adjective+noun, verb+noun, or with a sequence linked to de or d'. To write them correctly, one must observe the head, the agreement of the determiner, and the variations established by usage.