An accelerometer is a device that measures acceleration. Linear acceleration is a one dimensional motion and can be thought of as a train on tracks where movement is confined along a single axis. Most accelerometers today measure acceleration along all 3-axes; X, Y and Z. Units for acceleration are meters per second squared and are often expressed in terms of gravitational acceleration, g, where 9.81m/s² is equal to 1g. An accelerometer at rest on a table experiences 1g in the Z direction. This is important to note for applications where static tilt or inclination angle data is needed. As seen in the bottom figures on this slide, accelerometers can experience rotation about the Y-axis; known as roll. When roll occurs, the amount of acceleration on both the X-axis and Z-axis changes.