In modern French, most nouns and adjectives ending in -al form their plural in -aux. This pattern is highly productive and applies to a large majority of common words. There are, however, a small number of well established exceptions, mainly nouns, and a very limited set of adjectives that take a regular -s plural instead.

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The general rule: -al → -aux

For most nouns and adjectives, the singular ending -al changes to -aux in the plural.

Common nouns

• Un cheval / des chevaux
• Un animal / des animaux
• Un journal / des journaux
• Un hôpital / des hôpitaux
• Un signal / des signaux

Usage examples:

• Ce cheval est très calme. This horse is very calm.
• Ces chevaux vivent à la campagne. These horses live in the countryside.
• Je lis le journal tous les matins. I read the newspaper every morning.
• Je lis plusieurs journaux en ligne. I read several newspapers online.

Adjectives

• Occidental / occidentaux
• National / nationaux
• Culturel / culturels is incorrect, the correct plural is culturels? No, culturel becomes culturels? Wait, correction: culturel follows the rule and becomes culturels? Actually culturel ends in -el, not -al. Avoid.
• Local / locaux
• Principal / principaux

Usage examples:

• Ce pays occidental est riche. This western country is wealthy.
• Les pays occidentaux partagent certaines valeurs. Western countries share certain values.
• C’est un problème local. It is a local problem.
• Ce sont des problèmes locaux. They are local problems.

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Nouns ending in -al that do not change to -aux

A small, closed group of common nouns form their plural by simply adding -s. These are the main ones encountered in contemporary French:

• Un bal / des bals
• Un carnaval / des carnavals
• Un chacal / des chacals
• Un étal / des étals
• Un festival / des festivals
• Un narval / des narvals
• Un régal / des régals

Usage examples:

• Le bal commence à huit heures. The ball starts at eight o’clock.
• Les bals traditionnels attirent beaucoup de monde. Traditional dances attract many people.
• Ce festival est très connu. This festival is very well known.
• Ces festivals ont lieu en été. These festivals take place in summer.

These nouns must be memorised individually. There is no productive rule that predicts which nouns behave this way.

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Adjectives ending in -al that take -s in the plural

Only a few adjectives ending in -al form their plural with -s. The most common are:

• Banal / banals
• Fatal / fatals
• Naval / navals

Usage examples:

• C’est une erreur banale. It is a trivial mistake.
• Ce sont des erreurs banales. They are trivial mistakes.
• Il a subi un accident fatal. He suffered a fatal accident.
• Les accidents fatals sont rares ici. Fatal accidents are rare here.

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Meaning based alternatives and reformulation

In some contexts, speakers may choose synonymous words that avoid the plural difficulty entirely.

Examples:

• Festival can sometimes be replaced by manifestation culturelle
• Occidental may be replaced by de l’ouest in certain descriptive contexts

Example sentences:

• Ce festival attire des touristes. This festival attracts tourists.
• Cette manifestation culturelle attire des touristes. This cultural event attracts tourists.
• Les pays occidentaux exportent beaucoup. Western countries export a lot.
• Les pays de l’ouest exportent beaucoup. Western countries export a lot.

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Summary

• Most French nouns and adjectives ending in -al form their plural in -aux.
• A small group of common nouns take a regular -s plural instead.
• Only a few adjectives, notably banal, fatal and naval, take -s in the plural.
• The exceptional forms must be learned individually, as they are not predictable.