Extensometry is composed of all the techniques available for the measurement of the local state of STRAIN at one or various points of the surface (or in the volume) of a structure. The behaviour of the structure submitted under a sollicitation, is characterized by the 3D state of STRESS and the CONSTITUTION LAWS of the material. This state of stress is calculated from the strain measurements related to the stress components by those laws of constitution. At each point the deformation is not the same following any direction and the total information on deformation requests a sufficient number of strain measurements. LONGITUDINAL and SHEAR strain components are disponible for this, but generally only longitudinal strain c are measured: as shown on Figure 1 a longitudinal strain ε is a relative displacement between two points.