Trapeziometacarpal instability is a well-known condition that could lead to trapeziometacarpal osteoarthritis. With the focus in the literature being on ligament insufficiency, bony deformity has received much less attention. Although trapezial tilt is currently used to measure trapezial dysplasia, we believe trapezial inclination is a more reliable measurement. The purpose of this study is to compare these two methods for evaluating trapezial dysplasia and to define the most reproducible measurement. Fifty patients were identified who had three consecutive radiographs of the trapeziometacarpal joint. Eaton views with little to no signs of osteoarthritis (Eaton stage I or II) were used. Both trapezial tilt and trapezial inclination were measured by two independent observers on three radiographs for every patient. Intra-observer variation, absolute agreement and consistency, and intra-observer variation and consistency were assessed for both measurements. Mean trapezial tilt was 135° and mean trapezial inclination was 10°. For both observers, intra-observer variation of trapezial inclination was significantly less than for trapezial tilt. Our study shows that mean trapezial inclination in a middle-aged adult population is approximately 10°. This measurement is more reproducible than the more frequently used trapezial tilt and may be a better parameter to define trapezial dysplasia and to guide its treatment.