In the 17th edition of the Chicago Manual of Style, §8.159, the following rules are given (among others) for headline-style capitalization:
- Capitalize the first and last words in titles and subtitles (but see rule 7), and capitalize all other major words (nouns, pronouns, verbs, adjectives, adverbs, and some conjunctions—but see rule 4).
- Lowercase the articles the, a, and an.
- Lowercase prepositions, regardless of length, except when they are used adverbially or adjectivally (up in Look Up, down in Turn Down, on in The On Button, to in Come To, etc.) or when they compose part of a Latin expression used adjectivally or adverbially (De Facto, In Vitro, etc.).
- Lowercase the common coordinating conjunctions and, but, for, or, and nor.
But then, in §8.160, the following example is given:
Four Theories concerning the Gospel according to Matthew (2,3)
In understand that the is lowercased because of Rule 2 and to is lowercased because of Rule 3, but why are concerning and according lowercased? Aren't these “major words”?