Two different interpretations of the effective diameter are in use in various countries, mainly as a result of differing techniques of measurement, and the difference between these interpretations is discussed. The importance of pitch errors in relation to their effect on the fit of mating threads is described, together with the concept of the virtual effective diameter as a means of establishing systems of limits and tolerances for screw threads.The requirements for assembly of screws and nuts are given and the calculation of the virtual effective diameters of screw threads is discussed in relation to the different interpretations of the effective diameter.Finally, the relation between the two interpretations and the associated measuring techniques is discussed. It is shown that the common definition of effective diameter adopted by the United Kingdom and the U.S.A. is that best suited to their gauging and measuring techniques.