How do GMD and AMD differ?

I am studying self and mutual inductance now.
I have two unknow nouns,GMD and AMD.
GMD=Geometric Mean Distance
AMD=Arithmetic Mean Distance
What are they different ?
Any illustration explain them ?

TKS

http://weewave.mer.utexas.edu/MED_fi...C_ths/body.pdf
See pages 9+ ..

Regards,
IanP