Digitalisation of silicon carbide particles at different time intervals(Figure 9) around the S-C' fabric allow us to monitor movement of particles and to calculate the local deformation tensor F with the program "marker analysis" (Bons et al. 1993). The program superimposed a regularly spaced polar grid on the sample. The grid origin was at the rotation centre of the upper glass slide and a second marker point in the annular shear zone was chosen to fix a second reference axis. The position of each marker point at time (t) is defined by its distance to the origin and its polar angle to the reference particle. The position of each grid node is defined with respect to four adjacent marker particles occupying at least three different quadrants. A deformed polar grid can be constructed with the technique of a least squares best fit method which determine the displacement of each grid node from the displacement of the four adjacent marker particles. Our experiment is virtually two-dimensional and a deformation tensor F at any point can be fully described by four independent coefficients, whose values depend on the chosen reference frame. The tensor can be expressed by deformation parameters such as finite strain R_f, a finite 'mean' vorticity number W_n (Passchier 1988) and area change A_f. A_f is 1 for absence of dilation and can represent area decrease of area increase (A_f.>1). W_n is a measure of the rotational component of the deformation normalised for strain (Truesdell 1953, Passchier 1988). Positive vorticity numbers indicate a bulk clockwise rotation. W_n is 0 for a pure shear deformation, 1 for a dextral simple shear deformation and W_n1 indicates a rigid body rotation.