Pro-sentences are sometimes seen as grammatical interjections, since they are capable of very limited syntactical relations. But they can also be classified as a distinct part of speech, given that (other) interjections have meanings of their own and are often described as expressions of feelings or emotions.
In some languages, the equivalents to yes and no may substitute not only a whole sentence, but also a part of it, either the subject and the verb, or the verb and a complement, and can also constitute a subordinate clause.
The Portuguese word sim (yes) gives a good example:
In some languages, such as English, yes rebuts a negative question, whereas no affirms it. However, in Japanese, the equivalents of no (iie, uun, (i)ya) rebut a negative question, whereas the equivalents of yes (hai, ee, un) affirm it.
Some languages also have a specific word which rebuts a negative question. German has "doch"; French has "si" (not to be confused with the Spanish word "sí", meaning "yes"). Neither has a clear English translation.
The prosentential theory of truth developed by Dorothy Grover, Nuel Belnap, and Joseph Camp, and defended more recently by Robert Brandom, holds that sentences like "p" is true and It is true that p should not be understood as ascribing properties to the sentence "p", but as a pro-sentence whose content is the same as that of "p." Brandom calls " . . .is true" a pro-sentence-forming operator.