Computed tomography scans of 22 cadaveric adult elbows were obtained in 3 forearm positions: full supination, neutral, and full pronation. The radial head dimensions, the radiocapitellar joints, and the proximal radioulnar joints were measured. Multivariate analysis of variance was used to determine which portions of each articulation were the most congruent. The results showed that the radial head tended to become uncovered at the radial lip (P < .001). The radiocapitellar joint was tighter in pronation than in supination (P = .001). The proximal radioulnar joint was most congruent at the middle proximal radioulnar joint, at the midportion and posterior aspects rather than the anterior aspect (P < .001). The proximal radioulnar joint coverage was between 69 degrees and 79 degrees . Prosthesis trial sizing should be judged by the articulations providing the most congruency: (1) the ulnar lip or trough of the radiocapitellar joint in pronation and (2) the posterior or midportion of the middle proximal radioulnar joint.