The ideal test should provide a region of pure, uniform shear stress within the test sample. The most simple test is the tension test on the (+/-45°)ns sample, where n is greater than one. The specimen width is usually between 13 and 25 mm and strain gauges are bonded to the sample at 0° and 90°. It can be shown that the shear stress in the sample, τ12, Where σx is the stress in the longitudinal direction (=P/A). The shear modulus, G12, is determined by plotting the shear stress against the shear strain and measuring the slope of the resulting curve.