"From 1996 to 2002, NGC Research Director, David W. Lange, wrote a series of columns for The Numismatist, the monthly publication of the American Numismatic Association. Each entry featured a different type of United States coin, and Dave shared with readers his insights into the technical and aesthetic qualities that defined it, while also providing helpful tips to guide collectors in selecting examples which best represented that particular design. These columns have now been thoroughly updated and are presented here for the benefit of NGC’s customers."
http://www.ngccoin.com/coin-grading-guide/default.aspx 
"From One to Seventy" NGC's David Lange 
Threaded Mode
