A basic question scheme that encompasses (1) the defendant's psychiatric status, (2) the effects of that status on his functioning, and (3) his apparent ability to participate in legal proceedings, is introduced to clarify the evaluation of fitness to stand trial. The way in which combinations of answers to three 'basic questions' generate a scheme that clarifies the difficulties encountered in most competency evaluations is shown. Eight paradigm cases are generated. Five of these (competence, incompetence, mentally ill but competent, malingering, and impaired but competent) are frequently straightforward. However, the three possibilities in which a defendant meets criteria entailed by two of the three questions are inherently subject to controversy. These situations (circumscribed psychosis related to the charges, malingering in the context of mental illness, and functional deficits in the context of mental illness, and functional deficits in the context of minor mental illness) are discussed in detail and illustrated with case material. 1 table and 11 references. (Author abstract modified)