In plane shear behaviour of textile performs is the most studied mechanical property because this mode of deformation is necessary for forming on double curvature surfaces. The mechanical behaviour which results is complex and specific considering the possibilities of relative movements between yarns and fibres. Two principal tests are used: the hinged framework or ``picture frame'' and the tensile test at 45 or ``bias test''. But the results are different because of different shear frame sizes. It is necessary to define a quantity characteristic of shear behaviour, independent of the measurement device, and that allows comparing two fabrics and the measures obtained by two laboratories on the same fabric. An experimental device is proposed which enables to measure the strain and tension during a picture frame test or a bias test. The analysis of the results on sample of different size is proposed in this paper.