Certified coins have been graded and sealed in a plastic holder. Since condition is a very important part of a coins value, it never hurts to have an independent opinion as to a coins condition. Several well regarded and generally accepted grading companies include PCGS (Professional Coin Grading Service), NGC (Numismatic Guaranty Corporation), ANACS (American Numismatic Association Certification Service) and ICG (Independent Coin Grading). Grading is a matter of opinion but these companies attempt to adhere to certain standards,using strike, luster, wear and contact marks to assign a grade to a coin. Uncertified coins on this site are graded by using A Grading Guide to United States Coins, published by the American Numismatic Association, of which I am a member (member #1135007, since 1987).