In this article some aspects of the `Borderline' concept are discussed. The argument focuses on the question: `What borders on what?' The descriptive aspects of psychiatry are related to its psychodynamic aspects by scrutinizing the relation between the concept `Borderline Personality Disorder' and the concept `Borderline Personality Structure'. It is stated that the two concepts refer to different sorts of psychopathology, which may be represented by an orthogonal set of axes. It is argued that the clinician, in order to determine correctly the indication for therapy, must be aware of both the descriptive and the structural psychopathology of the patient. `What borders on what?' has to be answered in the `horizontal' sense as well as in the `vertical' sense.