Yes and no

Yes and no, or similar word pairs, 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. Modern English uses 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,^{[clarification needed]} and sign language. Some languages, such as Latin, do not have yes-no word systems.

Answering a "yes or no" question with single words meaning yes or no is by no means universal. About half the world's languages typically employ an echo response: repeating the verb in the question in an affirmative or a negative form. Some of these also have optional words for yes and no, like Hungarian, Russian, and Portuguese. Others simply do not have designated yes and no words, like Welsh, Irish, Latin, Thai, and Chinese. 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 in response to 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 conventional parts of speech. Sometimes they are classified as interjections. 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. Yes and no are usually considered adverbs in dictionaries, though some uses qualify as nouns. 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.

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."

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.

Watts classifies yes and no as grammatical particles, in particular response particles. He also notes their relationship to the interjections oh and ah, which is that the interjections can precede yes and no but not follow them. Oh as an interjection expresses surprise, but in the combined forms oh yes and oh no merely acts as an intensifier; but ah in the combined forms ah yes and ah no retains its stand-alone meaning, of focusing upon the previous speaker's or writer's last statement. The forms *yes oh, *yes ah, *no oh, and *no ah are grammatically ill-formed. Aijmer similarly categorizes the yes and no as response signals or reaction signals.

Felix Ameka classifies these two words in different ways according to the context. When used as back-channel items, he classifies them as interjections; but when they are used as the responses to a yes–no question, he classifies them as formulaic words. The distinction between an interjection and a formula is, in Ameka's view, that the former does not have an addressee (although it may be directed at a person), whereas the latter does. The yes or no in response to the question is addressed at the interrogator, whereas yes or no used as a back-channel item is a feedback usage, an utterance that is said to oneself. However, Sorjonen criticizes this analysis as lacking empirical work on the other usages of these words, in addition to interjections and feedback uses.