In sign language, mouthing is the production of visual syllables with the mouth while signing. Although not present in all sign languages, and sometimes not in signers at all levels of education, where it does occur it may be an essential (that is, phonemic) element of a sign, distinguishing signs which would otherwise be homophones; in other cases a sign may seen to be flat and incomplete without mouthing even if it is unambiguous.
Mouthing often originates from oralist education, where sign and speech are used together. Thus mouthing may preserve an often abbreviated rendition of the spoken translation of a sign. In educated Ugandan Sign Language, for example, where both English and Ganda are influential, the word for very, Av", is accompanied by the mouthed syllable nyo, from Ganda nnyo 'very', and abuse, jO*[5]v", is accompanied by vu, from Ganda onvuma. Similarly, the USL sign finish, t55bf, is mouthed fsh, an abbreviation of English finish, and deaf, }HxU, is mouthed df.
However, mouthing may also be iconic, as in the word for hot (of food or drink) in ASL, UtCbf", where the mouthing suggests something hot in the mouth and does not correspond to the English word "hot".
Mouthing is an essential element of cued speech and simultaneous sign and speech, both for the direct instruction of oral language and to disambiguate cases where there is not a one-to-one correspondence between sign and speech. However, mouthing does not always reflect the corresponding spoken word; when signing 'thick' in Auslan, for example, the mouthing is equivalent to spoken fahth.