U+1F645 Face with No Good Gesture

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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, 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.