maybe this is another USA -vs- the world thing? in europe, ET numbers denote negative offset (ie how far under the arch the wheel sits) in the states it may mean a different thing.
vujades backspacing explanation is correct, although again, "backspacing" is solely used in teh US, nowhere else as far as i know, uses backspacing as a measurement.
tomato, tomato, lets call the whole thing off.
