This paper presents an explanatory overview of a large and complex grammar, DIAGRAM, that is used in a computer system for interpreting English dialogue. DIAGRAM analyzes all of the basic kinds of phrases and sentences and many quite complex ones as well. It is not tied to a particular domain of application, and it can be extended to analyze additional constructions, using the formalism in which it is currently written. For every expression it analyzes, DIAGRAM provides an annotated description of the structural relations holding among its constituents. The annotations provide important information for other parts of the system that interpret the expression in the context of a dialogue. DIAGRAM is an augmented phrase structure grammar. Its rule procedures allow phrases to inherit attributes from their constituents and to acquire attributes from the larger phrases in which they themselves are constituents. Consequently, when these attributes are used to set context-sensitive constraints on the acceptance of an analysis, the contextual constraints can be imposed by conditions on dominance as well as conditions on constituency. Rule procedures can also assign scores to an analysis, rating some applications of a rule as probable or as unlikely. Less likely analyses can be ignored by the procedures that interpret the utterance. In assigning categories and writing the rule statements and procedures for DIAGRAM, decisions were guided by consideration of the functions that phrases serve in communication as well as by considerations of efficiency in relating syntactic analyses to propositional content. The major decisions are explained and illustrated with examples of the rules and the analyses they provide. Some contrasts with transformational grammars are pointed out and problems that motivate a plan to use redundancy rules in the future are discussed. (Redundancy rules are meta-rules that derive new constituent-structure rules from a set of base rules, thereby achieving generality of syntactic statement without having to perform transformations on syntactic analyses.) Other extensions of both grammar and formalism are projected in the concluding section. Appendices provide details and samples of the lexicon, the rule statements, and the procedures, as well as analyses for several sentences that differ in type and structure.