Accelerometer

An accelerometer is a sensor that measures the proper acceleration or g-force of a device.

In virtual and augmented reality, accelerometers are often part of IMUs used for rotational tracking. They are used to track the rotational movements of pitch, yaw, and roll.

Unlike coordinate acceleration (rate of change of velocity), proper acceleration takes gravity into account (accelerometer resting on the sea level of earth would measure 1g or 9.78 m/s^2 upwards). Measuring the acceleration of gravity allows the smartphone to know whether its held in portrait or landscape mode.