A range of techniques that are used to obtain domain knowledge about an application for the purpose of constructing an expert system. Knowledge acquisition covers all forms of knowledge and any methods by which they may be obtained. Various aids have been developed, such as KADS, a major system that consists of both a methodology and a set of tools. Knowledge elicitation is a subfield of knowledge acquisition concerned with systematic procedures for gleaning knowledge from human experts. Many tools are borrowed from psychology; for example, repertory grids are used to help draw out discriminations between classes of data items.