ESDU 89029 provides a simple semi-empirical correlation of experimental data from the literature for installed tailplane efficiency for both fin- and body-mounted cases. Efficiency here is installed lift-curve slope normalised by the theoretical free-air value, in this case as predicted by ESDU 70011. The correlation yields two graphs, one for each case, of efficiency against the most appropriate geometric parameter characterising the airframe interference. Subcritical Mach number effects are accounted for by use of the compressible lift-curve slope from ESDU 70011 but it is found that some improvement in correlation could be achieved by an additional correction factor. The methods apply to cases with or without high-lift devices deployed. Except for fin-mounted tailplanes on close-coupled configurations, the methods will predict lift-curve slope to within 6 per cent for any straight-tapered tailplane, as is shown by a sketch of predicted against experimental results for some 70 cases. Two detailed worked examples illustrate the use of the methods. The techniques used to extract the installed lift-curve slope from the wind-tunnel data are detailed.