What exactly does the iPhone accelerometer measure?

Question

The apple documentation for UIAcceleration class says,

\"When a device is laying still with its back on a horizontal surface, each acceleration event has

Answer 1

It measures the gravity. The unit is chosen so that the gravity, 9.81 m/s^2, equals 1 unit. The sign tells how the phone axis is directed. In other words, what the phone considers downwards.

The phone measures 0 as acceleration in free fall. I don't know how much you want to throw your phone up and down to test it :)