Gives a method for determining the in-plane shear modulus (G12) of fibre-reinforced plastic composites with the use of a standard plate specimen. When applied to isotropic materials, the shear modulus measured is independent of direction. It is used to determine the shear modulus of test specimens but not to determine the shear strength. Applicable to a plate supported on two points on one diagonal and loaded on the other diagonal by the simultaneous movement of two loading points attached to a cross-beam. Suitable for use with fibre-reinforced plastic composites with both thermoset and thermoplastic matrices.