Although the mass-and-spring system of Fig. 5 is a useful mathematical model for a seismometer, it is incomplete as a practical design. The suspension must suppress five out of the six degrees of freedom of the seismic mass (three translational and three rotational) but the mass must still move as freely as possible in the remaining direction. This section discusses some of the mechanical concepts by which this can be achieved. In principle it is also possible to let the mass move in all directions and observe its motion with three orthogonally arranged transducers, thus creating a three-component sensor with only one suspended mass. Indeed some historical instruments have made use of this concept. It is however difficult to minimize the restoring force and to suppress parasitic rotations of the mass when its translational motion is unconstrained. Modern three-component seismometers therefore have separate mechanical sensors for the three axes of motion.