Do you think you have a Doubled Die?
One question that comes up frequently is whether something is a doubled die or just machine (or mechanical, or ejection) doubling. These are two really good guides for answering that question. Since a picture is worth a thousand words, I'll start with this diagram by Charles Daughtrey (coppercoins.com) illustrating the difference between machine doubling and a doubled die (called "hub doubling" in the diagram).
I'll follow that with this link to a monograph by JT Stanton about doubled dies and their evil mimics. JT wrote this years ago, and it's still an outstanding article about the differences. I include it as an appendix to the class notes of the Morgan Dollar class I teach at the ANA Summer Seminar.
If this is question encountered frequently enough that the mods think this is worth pinning in this forum, great. If not, that's fine, too.
Professional Coin Imaging • Prime Number Set • World Coins in Early America • British Trade Dollars