ESDU 91009 provides a method that is based on determining the effective (increased) aspect ratio of the tailplane due to the end plate effect of the fins. ESDU 70011 is then used to estimate the lift-curve slope for that effective aspect ratio. Wind-tunnel data drawn from the literature for a wide range of configurations were used to correlate the effective aspect ratio (determined using ESDU 70011) as a function of the ratio fin height/tailplane span and fin spanwise position. The data were correlated to within 5 per cent and apply to straight-tapered tailplanes of taper ratio between 1 and 0.4 (tip chord/centreline chord) and mid-chord sweep up to 45 degrees, for any fin planform, and are independent of fin vertical location on the tailplane. For swept tailplanes the correlation only applies when the fin chord extends well forward of the tailplane leading-edge (by at least 0.25 chord). It is restricted to attached flow at subcritical speeds. The ranges of geometries for the test data used in the correlation are tabulated, sketches show plots comparing predicted with experimental results, and a worked example illustrates the use of the method.