This paper is concerned with providing automatically generated online explanations to the user of a user interface or "tool" about what the interface can and cannot do, and what parameters or options are available or required with a given command. The COUSIN interface system, developed at Carnegie-Mellon, supplies a cooperative tool-independent user interface for tools whose objects, operations, input syntax, and display formats are declaratively represented in a "tool description" database. Explanations are produced automatically from this database in a single uniform style, taking the form of a fine-grained, tightly linked network of text frames supported by a ZOG menu-selection system. Exactly what information the net building (NB) program extracts from a tool description and the format in which this information is presented in the text frames is controlled by a second declarative database called the "aspect description" database. The declarative nature of aspect description makes it easy to adapt NB to changes in and extensions to the tool description formalism, and to experiment with the structure of the explanation network. A description of how the appropriate network frame can be found and displayed in response to specific explanation requests from the user is provided. (Author/ESR)