It has long been a tenet of systemic functional linguistics that lexis should be treated as "most delicate grammar", that is, that the "senses" of lexical items should be modelled in system networks that are essentially similar in kind to those used in model meanings realized in grammar. After verbs and nouns, adjectives are the most important lexical word class. This is a descriptive and theoretical study of this central grammatical category, providing a demonstration of the validity of Halliday's hypothesis, and aiming to contribute to the linguistic theory of both system.