A translational or rotational ground motion sensor sensitive to accelerations, velocities or displacements affixed and referenced to some stated frame (generally to the non-inertial frame of the rotating, gravitating Earth or a built structure), and generally measuring one of the six degrees of freedom of whole-body motion. Inertial sensors respond to accelerations but, because of the inductive readout method, often output a signal proportional to ground-motion velocity or ground motion. A well built inertial sensor has a bob moving "freely" in only one degree of freedom and is sensitive to accelerations for signal frequencies (much) smaller than the eigenfrequency of the sensor or displacements for frequencies (much) larger than the sensor's eigenfrequency in that degree of freedom (See: Fig. 5.3 upper left and lower left in Chapter 5 of this Manual). Because of hinging effects, very often inertial sensors intrinsically mix the linear and angular acceleration. To make sure that this does or does not happen, one has to examine the hinging mechanism of the sensor