ESDU 91031 presents a method for estimating the derivatives based on apparent mass calculations that allow for the interference effects of body, wing and tailplane on the fin. The method is modified as a result of comparisons with experimental data drawn from the literature for mid and low wing or tailplane position for which the theory tends to overestimate the sideforce. The method factors the lift-curve slope of an isolated surface for the interference effects, and values for that may be found from ESDU 70012. To establish the yawing moment, the sideforce is assumed to act at the aerodynamic centre, which is also predicted using ESDU 70012. The method applies to a single fin in the plane of symmetry and body-mounted tailplane; however, recommendations are made on how it may be extended to treat fin-mounted tailplanes which in fact only make small contributions to these derivatives. It applies to angles of attack less than five degrees and angles of sideslip less than four degrees, but the behaviour of the derivatives up to angles of attack of 20 degrees is illustrated in a particular case and compared with predictions from an extended theory. Sketches show the comparison of predicted with experimental results drawn from the literature; agreement is found to be within 15 per cent, which is in any case the anticipated tolerance on values of lift-curve slope from ESDU 70012. The method is extended to cover twin fin configurations.