Third-party grading (TPG) refers to coin grading & banknote grading authentication, attribution, and encapsulation by independent certification services.
These services will, for a tiered fee depending on the value of the coin, "slab" a coin and assign a grade of 1-70 on the Sheldon grading system, with 1 being the lowest grade, with only faint details visible to 70, a practically perfect, uncirculated coin with no scratches or wear. If a coin is determined to have been cleaned, altered or damaged in the past, the coin will still be given a general grade, such as Good or Very Fine, but will not be given a numerical rating on the Sheldon scale. These coins are commonly referred to as “details” coins and usually have lower value to collectors.
American Numismatic Association Certification Service (ANACS) 1979, Professional Coin Grading Service (PCGS) 1986, Numismatic Guaranty Corporation (NGC) 1987, and Independent Coin Graders (ICG) 1998, are the most popular and well-known services. Together they have certified over 100 million coins.
Certified Acceptance Corporation (CAC) is a coin certification service which evaluates certain high-end coins already certified other firms and assigns a CAC approval to the coin's slab if it meets certain standards based on the coin's eye appeal, strike and visual appearance. Coins with the CAC sticker are usually valued higher than those without CAC approval.