Third Party Grading (TPG) refers to coin grading & banknote grading, grading of stamps, cards, comic books, etc., authentication, attribution, and encapsulation by independent certification services. These services will, for a tiered fee, "slab" an item. Professional Coin Grading Service (PCGS), Numismatic Guaranty Corporation (NGC), Independent Coin Graders (ICG), and ANACS are the most popular and credible services. Together they have certified over 80 million coins. All four firms guarantee the grades and authenticity of their certified coins. Certified Acceptance Corporation (CAC) is a coin certification service which certifies—and makes a market in—certain high-end coins already certified by PCGS or NGC.