Accelerometers In Cell Phones, What's the frame of reference?

There are articles on the web that say many mobile devices have "accelerometers" in them. Are these accelerometers in usual terminology of physics? With respect to what frame of reference do they measure the acceleration?

Accelleration doesn't really have a baseline reference as such, because accelleration is a change, not a hard datum.

The frame of reference is typically 1g. In other words, the normal acceleration due to gravity that the phone (and everything else) experience when sitting "still".

Some laptops have this feature. If g disappears, its in free fall and it parks the HD to avoid damage

Are they directional?

3 axis. Google
3 axis accelerometer

"The 3-Axis Accelerometer has three separate internal accelerometers"
http://www.vernier.com/products/sensors/ac...ometers/3d-bta/

So they are directional, just usually sold as a triad unit then?

Thanks.

Yeah you can get bi-axial or tri-axial accelerometers and they are often in tiny packages like QFN (14 pins or so). Some packages give you an analogue readout for each channel, whilst others have I2C or SPI interfaces.