ESDU Aero C.04.01.00 introduces methods given in the series for estimating the linear change in hinge moment due to a change in angle of attack or due to a control or tab deflection. It is noted that the rates of change of hinge moment with angle of attack or control deflection are only constant over a limited range and strictly the slopes only apply through zero angle of attack or zero control deflection; however it is found that the results of using the methods may be treated as mean values over a reasonable range unless there is a large balance exceeding 50 per cent or the trailing-edge angle exceeds 30 degrees. Outside the linear range the moments increase much more rapidly. The effects on the range of linearity of profile shape, control chord and gaps are discussed and, in particular, the effect of nose blunting is illustrated. Some guidance on the range of linearity is provided. The methods are for two-dimensional flow but their application to finite surfaces, which depends on the type of balance used, is discussed. They apply in wholly subsonic flow but the effects of compressibility may be allowed for using the Prandtl-Glauert factor; it is not possible to treat the effect of shock waves. All the ESDU documents that provide the estimation methods are referenced.