http://www.amazon.com/The-mod

In reply to Type weights theory?:

http://www.amazon.com/The-modification-letterforms-Stanley-Hess/dp/09101...

In practice, I design the Regular weight first, then draw Extra Bold by fattening up the Regular with various FontLab tools (some global actions, some cut and paste, whatever it takes). Then I draw the Thin as a stroked monoline (over the Regular in the mask layer). Then I interpolate the other weights between these three instances, with a final manual clean up. Some people use the Superpolator app for interpolating, rather than Blend in FontLab.