1951, Gordon M. Messing, "Structuralism and Literary Tradition," Language, volume 27, number 1, page 3:
Bally remarks in passing, as Hall does not, that the inversion in toujours est-il que is part of a set phrase and hence invariable.
2009, Bryan A. Garner, Garner on Language and Writing, page 525:
American courts sometimes use cherry in place of apple, but the latter fruit vastly predominates. The American version is that rare set phrase that is not so well set, variations on the phrase being more common than the main phrase itself.
1992, Stanislaw Baranczak, "How to Translate Shakespeare's Humor?: (Reflections of a Polish Translator)", in the Performing Arts Journal, volume 14, number 3, page 83:
If it proves clearly unfeasible to make the audience laugh at a thin and far-fetched joke, it is always better to change the way the joke works . . . for instance, a pun based on the speaker's taking literally some set phrase or metaphor with a pun based on phonetic similarity.
Specific kinds of set phrase may include: idioms, whose meanings cannot be determined from their parts; proverbs, whose meanings can be derived from their parts and which express practical wisdom and often take the form of complete sentences; and catchphrases, set phrases associated with a specific person or group. Most idioms and proverbs are not set phrases, however, as significant variation is often possible. Also many idiomatic expressions do not form single units but allow intervening terms.
Set phrases are often clichés or colloquialisms, but these terms may include expressions other than set phrases.