In "Dusky v. United States" (1960), the U.S. Supreme Court articulated 3 abilities that determine a criminal defendant's competence to stand trial: He or she must be able to consult with counsel, have a factual understanding of the proceedings, and have a rational understanding of the proceedings. Although the legal determination of a defendant's competence involves a dichotomous judgment, the latent structures of the constructs that underlie the abilities articulated in "Dusky" are unknown. The current study focused on the "rational understanding" prong of the "Dusky" standard. We hypothesized that, whereas factual knowledge of the legal system and ability to assist counsel may fall on a continuum, plausible (i.e., rational) beliefs about legal proceedings may be dichotomous in nature. Taxometric analyses of the Appreciation scale of the MacArthur Competence Assessment Tool-Criminal Adjudication, with a sample of 721 defendants, provided support for a taxonic structure. (Contains 2 figures and 9 footnotes.)