Yes and no, or word pairs with similar words, are expressions of the affirmative and the negative, respectively, in several languages, including English. Some languages make a distinction between answers to affirmative versus negative questions and may have three-form or four-form systems. English originally used a four-form system up to and including Early Middle English and Modern English has reduced to a two-form system consisting of 'yes' and 'no'. It exists in many facets of communication, such as: eye blink communication, head movements, Morse Code, and sign language. Some languages, such as Latin, do not have yes-no word systems.
Some languages do not answer yes with single words meaning 'yes' or 'no'. Welsh, Irish, Finnish, Latin, and Chinese are among the languages that typically employ an echo response (repeating the verb with either an affirmative or negative form) rather than using words for 'yes' and 'no'. Such languages can also have words broadly similar to 'yes' and 'no'. Echo responses avoid the issue of what an unadorned yes means in response to a negative question. Yes and no can be used as a response to a variety of situations – but are better suited when asked simple questions. While a yes response to the question, "You don't like strawberries?" is ambiguous in English, the Welsh response ydw (I am) has no ambiguity.
The words yes and no are not easily classified into any of the eight conventional parts of speech. Sometimes classified as interjections, they do not qualify as such,[fact or opinion?] and they are not adverbs. They are sometimes classified as a part of speech in their own right, sentence words, or pro-sentences, although that category contains more than yes and no, and not all linguists include them in their lists of sentence words. Sentences consisting solely of one of these two words are classified as minor sentences.
Although sometimes classified as interjections, these words do not express emotion or act as calls for attention; they are not adverbs because they do not qualify any verb, adjective, or adverb. They are sometimes classified as a part of speech in their own right: sentence words or word sentences.[1][2][3]
This is the position of Otto Jespersen, who states that "'Yes' and 'No' ... are to all intents and purposes sentences just as much as the most delicately balanced sentences ever uttered by Demosthenes or penned by Samuel Johnson."[4]
Georg von der Gabelentz, Henry Sweet, and Philipp Wegener have all written on the subject of sentence words. Both Sweet and Wegener include yes and no in this category, with Sweet treating them separately from both imperatives and interjections, although Gabelentz does not.[5]