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.