Grammaticality refers to the grammatical well-formedness of a sentence. A sentence may be wellformed grammatically. In other words, it conforms to the grammatical rules of the language, but it is not necessarily semantically well-formed, that means it may not make sense at all because of the selectional restrictions. Certain words are not supposed to go together. For example: Green clouds are sleeping furiously. This sentence is grammatically perfect, but the problem with this sentence is that no one has ever seen any green clouds, and clouds never sleep, still less furiously. This sentence violates selectional restriction.