ESDU 97025 gives similarity rules for pressure coefficient. For subsonic and supersonic two- and three-dimensional flow over thin sections and for axisymmetric flow they are derived from linearised (potential) flow equations. For transonic flow they are derived from a non-linear equation for two- and three-dimensional flow only and no treatment for axisymmetric flow is given. Manipulation of the two-dimensional transonic rule to forms more readily applicable is illustrated. Such rules are valuable in providing a simple theoretical framework within which to work when formulating generalised databases from ad hoc experimental data. The application of the rules to drag, lift and pitching moment coefficients is shown. To illustrate the value of the rules, they are applied in an illustrative example to correlate sets of experimental results for the pressure drag of a wedge in both subsonic and supersonic flow, collapsing the data into a family of curves well.