ESDU Aero C.01.01.04 states that the rate of change of the lift coefficient curve with control deflection for a surface with a control with a gap is given by the product of its value when there is no gap and the ratio of the lift-curve slopes with and without gap multiplied by a factor, f. Curves plot the ratio of the lift-curve slopes for various gap sizes and chordwise positions of the gap for two-dimensional flow and for a wing with an aspect ratio of 3; linear interpolation between these two sets of curves with the reciprocal of aspect ratio is suggested. The gap sizes for blunt-, circular- and elliptical-nosed controls are defined in sketches. The factor, f, is plotted against balance, that is the ratio of the areas of the control forward and aft of the hinge line, for each of the nose shapes. Those curves apply to a gap greater than 0.004 times the aerofoil chord and for values less than that it is suggested linear interpolation with (1 - f) be used. The rate of change of the lift coefficient curve with control deflection for a surface without a gap may be found from ESDU Aero C.01.01.03. The method applies to full span controls with deflections up to 10 degrees, flow without separation, and for values of the ratio of control chord to aerofoil chord between 0.25 and 0.4. It is suggested that hinge bracket cut outs be ignored where the clearances are of the same order as the gap. A worked example illustrates the use of the data.