Certified Guaranty Company
Comic Guaranty LLC, also known as CGC is an independent member of the Certified Collectibles Group of companies. It is the first independent, impartial, expert third party grading service for comic books. The company was launched in early 2000 and has since gone on to become an important part of the comic book collecting community.
Comics graded by CGC are placed in an inner well (a sealed Barex sleeve, similar to Mylar), then sonically sealed in a hard plastic, tamper-evident holder (the process is often referred in slang as "slabbing") with a label at the top indicating the grade, page quality and any notes.
Comic books are graded on a scale from 0.5-10; CGC's grading scale can be read here.
CGC also offers a Signature Series service for signed collectibles witnessed by their employees and authorized representatives at comic book conventions, comic book stores, and at private signings. Signature Series books would typically receive a yellow label (assuming no restoration is found). There are various label colors denoting different aspects of the book.
The most common label would be the "Universal" label which is blue (may also be red as this was used for a period for Modern comics, but phased out), and would be assigned to a book if it is a standard unrestored copy. A Blue label holder may contain a book that has had the following work done to it, as CGC does not consider these controversial procedures to be Restoration: pressing, dry cleaning (erasing pencil writing or surface grime with an eraser or other object), a very minor amount of glue or color touch on a golden age book published in or prior to 1950, or a book that was disassembled and reassembled undetectably. A restored book would receive a "Restored" label, which is purple--often called a "PLOD" by collectors, or Purple Label Of Death. A "Qualified" label which is green would be assigned to a book that would otherwise have graded higher if not for a certain defect which affects its technical grade drastically (such as a clipped coupon, clipped stamp or a large tear on the back cover of an otherwise high-grade book). The green Qualified label is also used for books that appear to bear creator signatures that were not verified by a representative of CGC at the time the book was signed. A book that has had its staples cleaned or replaced without any other restoration present would also receive a green Qualified label.
CGC is based out of Sarasota, Florida and sends representatives to most major comic book conventions. Comics can be submitted to CGC at conventions, directly or thru a comic book store that is registered with CGC. Many websites offer a 20% discount if you use the submission form of CGC Affiliates. You do NOT send your books to these dealers, you send them directly to CGC yourself.
CGC has received its share of criticsim. This is predominantly because their business decisions are dealer-motivated and not necessarily collector friendly, and their inner workings are shrouded in mystery. Example: it took 7 years to find out that CGC does not consider disassembly and reasembly of a comic to be restoration.