The displacement of each part of a seismometer’s frame is identical for a purely translational motion. However, in the presence of rotary motion the different parts of a seismometer’s frame will undergo different displacements. The definition of the sensitivity of the seismometer then requires the selection of a reference location on the seismometer’s frame to which the sensitivity is attributed. This location does not necessarily coincide with the hinge and can be selected arbitrarily. The appropriate choice is to attribute the output signal to the location of the point mass of the equivalent simple pendulum (or reduced pendulum), which usually lies within the seismometer’s casing. Rotations of the sensor about this location produce no output signal due to angular or centripetal acceleration. The sensor then appears sensitive to linear acceleration only.