American Sign Language (ASL) has grammar just like any other sign language or spoken language. Sign languages are produced in the visual-gestural modality, as opposed to the vocal-auditory modality of spoken languages, which has a unique impact on their grammars. The study of ASL structure dates back to William Stokoe, Dorothy Casterline, and Carl Croneberg in the 1960s. Since then, linguists have been describing patterns in ASL responsible for creating meaning at all kinds of levels, from the word, to phrases and sentences and beyond. Not all of these linguists ascribe to the same theoretical framework, using various theories, models, and analyses. Some analyze the language from a generative linguistics perspective, while others from a cognitive linguistics perspective.