One of the basic elements of AR, is the visualisation device. In most situations a modified HMD, that includes one or more front-facing cameras.
These HMDs, are typically composed by one or two LCD or OLED displays, sometimes a pair of earbuds or headphones, and an Inertial Measurement Unit (IMU).
The IMU can be composed by accelerometers, gyros and magnetometers, being the latter only available in some models.