ESDU Aero C.01.01.03 gives two graphs. The first plots the theoretical (in inviscid flow) rate of change of lift coefficient with control deflection against control chord to wing chord ratio for various values of thickness/chord ratio. The curves of the second allow the theoretical value to be corrected for the effects of viscosity since they give the ratio of the slope of the lift coefficient curve with control deflection in incompressible flow to that theoretical value. The curves are plotted against the ratio of control chord to wing chord. Each curve applies to a value of the ratio of the lift-curve slope in incompressible flow to that in inviscid flow and that ratio may be obtained from ESDU Aero W.01.01.05. The data are accurate to within 5 per cent and apply to controls with a sealed gap. The effect of unsealing the gap is treated in ESDU Aero C.01.01.04 and for controls of finite aspect ratio ESDU 74011 should be consulted.