Expanding from the introduction to Reliefs presented in the page, there are many sources describing the A, B, and C Reliefs for this stamp. However, two articles present this topic so well, there is absolutely no reason to do anything but post them below. Both articles were published in The 1851 Issue of United States Stamps: A Sesquicentennial, a superb out of print book published by The U.S. Philatelic Classics Society (USPCS) in 2006. Thankfully, the title page to the books states, "The contents of this book are fully protected by copyright. Collectors, dealers, and philatelic authors are hereby authorized to make use of the information contained in this book without permission of the publisher, provided that proper acknowledgement of the source is made." The USPCS is an organization you should join if you are into US Classic Stamps. With ALL credit to Richard Celler and Elliot Omiya for both articles. Enjoy!