Saddles - question

[RuthnMeg]

At what 'measurement / stage' or whatever you want to call it, does a narrow saddle become a medium saddle?
Does that sound dim? Is it so obvious that I don't know? Or is there even an answer for it?

Help please!!

 

[dumpling]

Taking a wild guess but I wreckon medium starts about 6". My show saddle's an extra wide and is about 8 1/2".

 

[RuthnMeg]

Thanks.
Can anyone else hazard a guess? What measurement is a narrow?