A sensor that responds to point ground motions or structural acceleration and usually produces a signal that can be recorded in terms of displacement, velocity, or acceleration of the seismometer mass with respect to the ground on which the seismometer is installed. A seismometer is usually a damped oscillating mass that is connected with a ground-fixed base and frame via a suspension, e.g., a spring (see pendulum with Figure 1). Thus the mass serves as an inertial reference for detecting and measuring seismic ground motion relative to the suspended mass. These relative motions are commonly directly recorded or transformed into an electrical voltage (see transducer) which was traditionally recorded analog on paper, film or magnetic tape. Nowadays, digital recording media are almost exclusively used. The seismometer record can mathematically be converted to a record of the absolute ground motion. For inertial seismometers see Chapter 5 and DS 5.1 of this Manual. Another, non-inertial type of seismometers are strainmeters (see IS 5.1 in this Manual). Until recently "seismometer" meant only a translational sensor that senses one of the three components of translational motions. More recently, a rotational seismometer may also be deployed to complement such translational seismometers and allow for measuring all six components of