An accelerometer is a device that measures proper acceleration, also called the four-acceleration. For example, an accelerometer on a rocket accelerating through space will measure the rate of change of the velocity of the rocket relative to any inertial frame of reference. However, the proper acceleration measured by an accelerometer is not necessarily the coordinate acceleration (rate of change of velocity). Instead, it is the acceleration associated with the phenomenon of weight experienced by any test mass at rest in the frame of reference of the accelerometer device. For an example where these types of acceleration differ, an accelerometer will measure a value of g in the upward direction when remaining stationary on the ground, because masses on earth have weight m*g. By contrast, an accelerometer in gravitational free fall toward the center of the Earth will measure a value of zero because, even though its speed is increasing, it is at rest in a frame of reference in which objects are weightless.