Few topics make even confident French speakers hesitate like the plural of compound nouns—especially those written with hyphens. Should you add -s to the first word, the second word, both, or neither? Why is it des arcs-en-ciel but often des sages-femmes? And why does après-midi seem to have two acceptable plurals depending on which reference you consult?
The reassuring news is that this is not chaos. It is the result of two systems coexisting:
- an older, “semantic” system that tries to pluralise only the parts that really refer to countable things, and
- a more regularising approach encouraged by the 1990 orthographic rectifications, which aim to make some compound plurals behave more like ordinary nouns.
Once you understand which families of compounds are affected by each system, many famous examples (including arc-en-ciel and après-midi) become predictable.
1) What counts as a “compound noun” in French?
A French compound noun is a single lexical unit made of multiple elements: arc-en-ciel, chef-d’œuvre, abat-jour, sage-femme, gratte-ciel, après-midi, etc. They often appear with hyphens, though modern spelling can vary. The difficulty is that French compounds come from different underlying patterns, and plural marking depends largely on that pattern.
A useful guiding principle is the traditional one: only nouns and adjectives “want” to take plural, while prepositions and verbs usually do not—unless a reform or modern standardisation rule says otherwise.
2) The “classic” logic: pluralise what carries the countable meaning
In the traditional approach taught in many schools, you pluralise the element(s) that represent what is actually being counted.
a) Noun + adjective / adjective + noun
These behave fairly intuitively because both parts are “declinable”:
- une sage-femme → des sages-femmes
- un coffre-fort → des coffres-forts
Here the noun is clearly countable (sage, coffre), and the adjective follows ordinary agreement patterns.
b) Noun + noun
These often take plural on both nouns when both are countable in meaning, but usage varies by item and dictionary tradition. Some compounds have become lexicalised and resist full regularisation in writing, so you often rely on the dictionary.
c) Noun + preposition + noun (arc-en-ciel, chef-d’œuvre)
A widely taught traditional rule is: only the first noun pluralises, because it is the “head” of the phrase:
- un arc-en-ciel → des arcs-en-ciel
- un chef-d’œuvre → des chefs-d’œuvre
This logic corresponds to an underlying interpretation: “arcs in the sky,” “chefs of work.” The second noun functions more like a complement than a second countable item.
This is why des arcs-en-ciel remains the standard plural in major references and teaching rules.
3) The special case that everyone argues about: après-midi
Après-midi is fascinating because it sits at the intersection of older practice and the 1990 rectifications.
- The Académie française entry notes that après-midi is traditionally treated as invariable in many uses, but adds that it may be written après-midis in the plural according to the 1990 rectifications.
- The OQLF explains the rectified rule for preposition + noun compounds with a hyphen: the plural marker is placed on the second element only, giving un après-midi, des après-midis.
- Le Robert explicitly acknowledges the coexistence: “Des après-midis ou des après-midi (invariable).”
So what should you do?
- If you want to follow the rectified, more regular pattern: des après-midis (plural -s on the final element) is fully legitimate and supported by the Académie’s own note and OQLF’s explanation of the 1990 rule.
- If you are reading older texts or writing in a style that keeps the traditional invariable form: des après-midi is still widely seen and explicitly mentioned as acceptable by some dictionaries.
In other words, après-midi is not a “trick question.” It is a genuine zone of norm variation.
If you want a structured way to master these choices (traditional vs rectified, dictionary norms, and the main compound patterns) through guided examples instead of memorising scattered rules, ExploreFrench’s online French grammar lessons are a good fit—because compound plurals only really stick when you practise them inside real noun phrases and sentences.
4) What the 1990 rectifications actually changed (and why)
The orthographic rectifications aimed to reduce inconsistency in several areas, including compounds. For certain families, the idea is simple: treat the compound more like an ordinary noun and mark plural in a predictable place.
The Académie’s official page on the rectifications states, for example, that in many hyphenated compounds the plural is marked only on the second element:
un pèse-lettre, des pèse-lettres; un abat-jour, des abat-jours.
The OQLF summarises the same point more explicitly for the two families most relevant here:
- verb + noun compounds: un essuie-main, des essuie-mains
- preposition + noun compounds: un après-midi, des après-midis
Crucially, the rectifications do not mean “pluralise everything everywhere.” They target specific structural types and propose a more regular plural placement.
They also include exceptions. For instance, OQLF notes that the rule does not apply the same way when the second element begins with a capital letter or contains a determiner, giving examples such as prie-Dieu or trompe-l’œil.
5) Why arc-en-ciel doesn’t follow the same “preposition + noun” rectification logic
At first glance, arc-en-ciel looks like noun + preposition + noun, so learners may expect the rectified rule to produce something like arc-en-ciels. But mainstream explanations of the rectifications themselves often note that not all “N + preposition + N” compounds were re-regularised in the same way, and in practice arcs-en-ciel remains the standard plural form.
That is an important lesson: the rectifications are not a single blanket formula. They reduce inconsistency in targeted families, but they coexist with long-established dictionary norms for many older compounds—especially those whose structure does not behave like the “verb + noun” or “preposition + noun” families the reform explicitly highlights.
So, for real-life writing:
- treat arcs-en-ciel as the safe standard;
- treat après-midis as a rectified standard option, with après-midi still frequently encountered.
6) The practical learner takeaway: learn compounds as patterns, then verify the classics
A realistic strategy is:
- Recognise the big families
- adjective+noun / noun+adjective: often both agree as expected
- noun+prep+noun: often only the first noun pluralises (arcs-en-ciel, chefs-d’œuvre)
- verb+noun and certain prep+noun compounds: rectifications favour plural on the last element (abat-jours, pèse-lettres, après-midis)
- Accept that a handful are lexicalised “famous exceptions”
Even with good rules, some compounds remain dictionary-driven. - Practise in context
Plurals are not just spelling; they appear inside determiners, adjective agreement, and real sentences. That’s why learners improve fastest when compounds are recycled across listening, reading, and writing tasks.
If you want that kind of structured recycling—where you repeatedly meet compounds inside everyday contexts (time phrases, objects, routines, descriptions)—ExploreFrench’s French vocabulary practice modules are a good complement: compounds become far less intimidating once they are learned as “chunks” you actually use rather than as isolated orthography puzzles.
Conclusion
French compound plurals look intimidating because they sit at the crossroads of meaning, history, and standardisation. But the system is not arbitrary:
- Many compounds follow a semantic logic: pluralise the element that carries the countable meaning (arcs-en-ciel).
- The 1990 rectifications regularise certain families by marking plural predictably on the final element (abat-jours, pèse-lettres, après-midis).
- Some high-frequency items (especially après-midi) legitimately show variation across references and registers.
Once you internalise the main compound families and the specific scope of the rectifications, most “tricky” plurals become manageable—and you’ll start seeing them as a small, learnable part of French spelling rather than a random minefield.