Background. Primary osteoarthritis is usually selected in either epidemiological or therapeutic studies. This implies exclusions. Among cases of secondary osteoarthritis considered for either stratification or exclusion - or for prognosis and treatment in daily practice - are those due to architectural defects. Parameters of the latter should be measured to ascertain diagnosis. At present, measurements have to be performed either on digitized reduced films or standard radiographs. Objective. To finalize an instrument capable of measuring the main angles and segments characteristic of the main dysmorphisms of the hip, knee and foot on different sizes of films. Methods. An arthrometer drawn on transparent material to be placed on radiographs was designed, involving several appropriate protractors and millimetric scales; it was tested on 60 hip, 35 knee and 17 foot radiographs with various architectural defects. Angles and segments most often used according to literature were measured. Reduction rates of films were various, reflecting the range of radiograph sizes currently used in everyday practice. Results. Measurements were easily performed on radiographs from standard (100%) up to 50% of reduction rate. So the arthrometer allows the recognition, especially in moderate, not obvious forms, of the following developmental or acquired dysmorphisms: hip congenital dysplasia and subluxation, including coxa valga and neck excess of anteversion; acetabular protrusion and coxa vara; tilt deformity; knee: patellar height abnormalities, patellar maltracking, trochlear depth insufficiency; foot: pes cavus, flat foot. Angle and segment ratios do not change in reduced film, whereas segments (absolute lengths) obviously should be converted according to the reduction rate for retrieving the classical values established for decades. Conclusion. The arthrometer allows us to measure the relevant parameters of various dysmorphisms without drawing lines over the films themselves. It is suitable to reduced as well to standard radiographs. However, only the instrument and its ability to be used in various films sizes are here presented. Study of reproducibility of measurements - especially concerning the joint space width - remain to be performed.