In a phone or small computer, the accelerometer is the tiny sensor that feels how you are holding the device. It notices tilt and movement, so the screen can turn the picture and the words the right way up — tall when you hold it tall, wide when you turn it on its side. That is the sense a learner meets: the thing that makes auto-rotate work. Under the name, it measures acceleration, including the pull of gravity, which tells it which way is down. Phones also use it for step counting and for games you tilt.