wheel question.. why is width and diameter in inches but

[TWFAUST at aol.com]

In a message dated 3/16/2005 3:21:01 PM Eastern Standard Time, 
quattro-request at audifans.com writes:

> Maybe the explanation lies in these facts:
> 
> 1- Offset exists only on wheels that are NOT zero-offset (where the bolting
> flange is in the center of the tread)
> 2- All RWDs (American cars until 1980) usually use zero-offset wheels 
> 3- FWD cars are an European invention (since the 30s with Citroen)
> 4- FWD cars need positive offset wheels so the load on the wheel bearings is
> in the center of the tire tread and the CV joint is not too far inboard.
> 
> So, my guess is that European manufacturer needed the offset for their FWD
> cars (and measured it in mm) but the tire standards were already cast in
> stone (in inches, more precisely). 
> Hence the dual standards of the automotive wheels.
> 
> FWIW,
> 
> Louis-Alain
> In Quebec where dual standards are everywhere. 
> I am 6'3", but my car fuel economy is around 12 liters/100km.
> 

Very reasonable. Now could someone tell me why offset is stated as "ET" as in 
ET 45. sometimes it is only an "E" as in E38.

Tom Faust