In relation to the design of an inertial sensor, a pendulum is either (1) a mass moving strictly in a translational manner (Figure 1, bottom), as in most simple geophones; or (2) a mass attached by a rigid arm to a pivot axis orthogonal to that arm and to the active axis of the sensor, as in many accelerometers and broadband velocity sensors (Figure 1, top). A mass moving on a simple cantilever, as in many MEMS devices, is equivalent to (2), whereas the restoration of classical pendulums derives solely from the gravitational force, with springs. They provide, either acting alone or in conjunction with gravity, the restoring force in common seismic pendulums. Nearly all pendulums are purposely damped by some means. The intended direction of (infinitesimal) mass motion relative to the instrument frame (thus the ground) is the active axis of the pendulum. Most seismic sensors are based on one or the other of these forms of constrained pendulums, although a few are based on magneto hydrodynamic and other phenomena. In the simple or common pendulum, the mass is below the pivot, and constrained to move in a vertical plane; in the inverted pendulum, the mass is directly over the pivot; and in the horizontal pendulum, the mass is to one side of the pivot, and is constrained to move in a nearly horizontal circle. See Chapter 5 (pp. 10-16 and Fig. 5.4- 5.7) in this Manual. Figure 1. Schematic diagrams of a mass-on-rod pendulum (top) and a translational pendulum (bottom). The mass-on-rod pendulum moves in a circular arc about its pivot, here pointing into the page, and is parallel to the putative active axis only while at zero deflection. The translational pendulum is constrained by its suspension to move only in the active-axis direction. Spring is shorthand for all mechanical and force feedback restoring forces in combination