Sign languages (also known as signed languages) are languages that use the visual-manual modality to convey meaning rather than spoken words. They are expressed through manual articulation in combination with non-manual markers, and are full-fledged natural languages with their own grammar and lexicon. Sign languages are not universal and are usually not mutually intelligible, although there are similarities among different sign languages. Wherever communities of deaf and hard of hearing people exist, sign languages have developed as a means of communication and form the core of local Deaf cultures. Although signing is used primarily by deaf and hard of hearing people, it is also used by hearing individuals, such as those with deaf family members including children of deaf adults (CODAs). Sign languages are distinct from body language, a type of nonverbal communication. Linguists also distinguish natural sign languages from other systems that are precursors to them or derived from them, such as constructed manual codes for spoken languages, home sign, baby sign language, and signs used with non-human primates. The number of sign languages worldwide is not precisely known. Each country generally has its own native sign language, and some have more than one. The 2021 edition of Ethnologue lists 150 sign languages, while the SIGN-HUB Atlas of Sign Language Structures lists over 200 and notes that there are more that have not yet been documented. As of 2021, Indo-Pakistani Sign Language is the most widely used sign language in the world, and Ethnologue ranks it as the 151st most spoken language in the world. Some sign languages have obtained some form of legal recognition.
History
Groups of deaf people have used sign languages throughout history. One of the earliest written records of a sign language is from the fifth century BC, in Plato's Cratylus, where Socrates says: "If we hadn't a voice or a tongue, and wanted to express things to one another, wouldn't we try to make signs by moving our hands, head, and the rest of our body, just as dumb people do at present?" Most of what is known about pre-19th-century sign languages is limited to the manual alphabets (fingerspelling systems) that were invented to facilitate the transfer of words from a spoken language to a sign language, rather than documentation of the language itself. Debate around European monastic sign languages developed in the Middle Ages has come to regard them as gestural systems rather than true sign languages. Monastic sign languages were the basis for the first known manual alphabet used in deaf schools, developed by Pedro Ponce de León.
The earliest records of contact between Europeans and Indigenous peoples of the Gulf Coast region in what is now Texas and northern Mexico note a fully formed sign language already in use by the time of the Europeans' arrival there. These records include the accounts of Cabeza de Vaca in 1527 and Coronado in 1541. In 1620, Juan Pablo Bonet published Reducción de las letras y arte para enseñar a hablar a los mudos ('Reduction of letters and art for teaching mute people to speak') in Madrid. It is considered the first modern treatise of sign language phonetics, setting out a method of oral education for deaf people and a manual alphabet.
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