Coin collecting can become a "competitive" activity, as prompted by the recent emergence of PCGS (Professional Coin Grading Service) and NGC (Numismatic Guarantee Corporation) Registry Sets. Registry Sets are private collections of coins verified for ownership and quality by numismatic grading services. The grading services assess collections, seal the coins in clear plastic holders, then register and publish the results. This can lead to very high prices as dedicated collectors compete for the very best specimens of, for example, each date and mint mark combination.