ESDU Struct 01.01.10 provides an equation from which the stress may be calculated at which initial torsional buckling occurs of a strut of any section in which the sides meet at a single point (for example, an angle or tee section). Bulb angle sections can also be treated by the method provided the bulb diameter is not more than three times the mean thickness, and it can also be used for stiffeners attached to a thin skin. Plasticity is taken into account by representing the stress-strain curve of the material by the expressions given in ESDU 76016, which require a material characteristic, m, and the stress, f, at which the tangent modulus is half the elastic modulus of the material in compression. Evaluation of the equation requires the shear and tangent moduli of the material, the torsion and secondary warping constants of the section, the polar moment of inertia of the section about its centre of rotation and its radius of gyration about an axis through its centroid parallel to its base, and the effective length of the strut. The torsion and warping constants may be determined using the data in ESDU Struct 00.07.01. The effective length depends on the end constraints and its calculation is discussed. The equation, which is also provided in graphical form, may also be applied to any open section strut of doubly symmetrical or point symmetrical section by replacing the secondary warping constant by the primary warping constant, which for Z-sections may be found using ESDU 77023. A worked example illustrates the use of the data.