Piezo-electric crystals are man-made or naturally-occurring crystals that produce a charge output when they are compressed, flexed or subjected to shear forces. The word "piezo" is a corruption of the Greek word meaning "squeeze." In a piezo-electric accelerometer, a mass is attached to a piezo-electric crystal and that, in turn, is mounted to the case of the accelerometer. When the body of the accelerometer is subjected to vibration, the mass mounted on the crystal wants to stay still in space, due to inertia, and so compresses and stretches the piezo-electric crystal. This force causes a charge to be generated and due to the Newton law of F=ma, and this force is, in turn, proportional to acceleration. The charge output is either converted to a low impedance, voltage output by the use of integral electronics (Example: in an IEPE accelerometer) or made available as a charge output (Picocoulombs /g) in a charge output, piezo-electric accelerometer.