Accelerometer and/or Gyroscope for gesture sensing
Consider an accelerometer experiencing a net acceleration of gx - gz.

There are an infinite possible ways to achieve this, here are few examples:


The sensor is oriented with z pointing towards the sky, and the user is moving it laterally relative to the ground with acceleration g.
The sensor is oriented x pointing towards the ground, and the user is moving it laterally relative to the ground with acceleration -g.
The sensor x/z plane is oriented at a 45 degree angle, and the user is moving it in a down-ward/left direction.
The sensor is rotating about some point, moving with a non-constant velocity such that the acceleration is gx - gz.


etc.

So a single accelerometer cannot determine its orientation if it's experiencing acceleration and gravity.