thin was actually an abritrary width.

Hmmmmm! I was taught that "thins" were the lower case i, upper case I and lower case l.
When counting points for a headline the em ( W/w and M/m) were counted as two points (the widest letters) en (average sizes of letters) as one point and the thins (I/i and l) as a half point. Then computers came in and made it all automatic and nobody cares anymore - sigh- I am feeling rather obsolete right now.