The paper investigates key issues pertaining to Natural Language Interface [NLI] development. The notion of natural language understanding interfaces (NLUIs) is introduced. Special attention is devoted to problems related to the specification of natural language’static’ semantics. The points we raise involve the applicability of attribute grammars to NLUI design. The considerations to be developed stem from the insight gained through experience drawn from the creation of a NLUI to a software called THALES. THALES is a program package for plane geometry constructions. Following a brief introduction to the principal modules of the interface, an outline of the linguistic features of the sublanguage is presented. A more thorough description of the semantic module is given. It is argued that attribute grammars are highly suitable for specifying and generating different modules of NLUIs. The idea of differentiating between static and dynamic semantics as a basis for capturing NLUI semantics is introduced.