How to Capitalize French Titles

Apple on top of stacked books
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The capitalization of titles and names in French and English is quite a bit different. In English, the first word in the title and all subsequent "important" words (everything except articles, conjunctions, and prepositions) are capitalized. The rules are more complicated in French - so complicated, in fact, that I have been unable to find a definitive system. I have found various levels of support for the following three schools of thought.

I.Standard capitalization
 Here, capitalization depends on the position and importance of the words in the title.
 A.The first word is always capitalized.
 B.If the first word is an article or other determiner, the first noun and any adjectives that precede it are capitalized.
   Trois ContesUn Cœur simple
   Le Petit RobertLe Nouveau Petit Robert
   Le Bon UsageLe Progrès de la civilisation au XXe siècle
 C.If the title consists of two words or phrases of equal value, they are considered "co-titles" and each one is capitalized according to the above rules.
  Guerre et Paix

Julie ou La Nouvelle Héloïse

This is the system that I learned in college and have used ever since. It is also used in Le Petit RobertLe Quid, and throughout the Dictionnaire de citations françaises.

Le Bon Usage briefly discusses an inconsistency in the capitalization of titles. It does not mention system I (above), but lists two others:

II.Important noun capitalization
 In this system, the first word and any "important" nouns are capitalized.
   Trois ContesUn Cœur simple
   Le petit RobertLe nouveau petit Robert
   Le bon UsageLe Progrès de la Civilisation au XXe siècle
Le Bon Usage states that system II is more common than III and uses it in its own bibliography.
III.Sentence capitalization
 In this system, only the first word of the title is capitalized.
   Trois contesUn cœur simple
   Le petit RobertLe nouveau petit Robert
   Le bon usageLe progrès de la civilisation au XXe siècle

A number of websites use this system, crediting it either to the MLA Handbook or to « normes ISO ». I was unable to find any official online documentation for either of these sources.

On the spines of the several dozen French novels I own (mostly published by Folio), there is a fairly even distribution between capitalization systems II and III.

Summary: Since even Le Petit Robert (the quintessential French dictionary) and Le Bon Usage(the French grammar bible) can't seem to agree about how to capitalize titles, it is clear that either there is no standard system or that the standard is not well known. Personally, I'm going to stick with what I learned in college.

Note: Of course, proper nouns are not affected by these capitalization systems; they always follow their rules.
IV.Capitalization of surnames
 French surnames are often capitalized in their entirety, especially in bibliographies and administrative documents.
  Gustave FLAUBERTCamara LAYE