Yes and no are two words for expressing the affirmative and the negative, respectively, in modern English. English originally used a four-form system up to and including Early Middle English but Modern English has reduced this to a two-form system consisting of just ‘yes’ and ‘no’. Some languages do not answer yes–no questions with single words meaning ‘yes’ or ‘no’. Welsh and Finnish are among several languages that typically employ echo answers (repeating the verb with either an affirmative or negative form) rather than using words for ‘yes’ and ‘no’, though both languages do also have words broadly similar to ‘yes’ and ‘no’. Other languages have systems named two-form, three-form, and four-form systems, depending on how many words for yes and no they employ. Some languages, such as Latin, have no yes-no word systems. The words yes and no are not easily classified into any of the eight conventional parts of speech. Although sometimes classified as interjections, they do not qualify as such, and they are not adverbs. They are sometimes classified as a part of speech in their own right, sentence words, word sentences, 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. The differences among languages, the fact that in different languages the various words for yes and no have different parts of speech and different usages, and that some languages lack a ‘yes-no’ word system, makes idiomatic translation difficult.