Mouth Morphemes

Non-manual mouth configurations in sign languages that carry grammatical or adverbial meaning independent of the words they accompany. In American Sign Language, mouth morphemes include patterns such as "mm" (relaxed, normal), "cha" (large), "oo" (small or close), "th" (careless or awkward), and "pah" (finally). They modify the manner, size, intensity, or duration of a sign and are not optional: omitting them changes meaning or produces ungrammatical ASL. Mouth morphemes are often missed in beginner ASL instruction and in computer-vision-based sign-recognition systems, making them a specific challenge for AI-based ASL learning tools and avatars.