In addition to the rating of coins by their wear, Proof coinage occurs as a separate category. These are specimens struck from polished dies and are often packaged and sold by mints. This is frequently done for Commemorative coins, though annual proof sets of circulating coinage may be issued as well. Unless mishandled, they will stay in Mint State. Collectors often desire both the proof and regular ("business strike") issues of a coin, though the difference in price between the two may be significant. Additionally, proof coins follow the same 1–70 grading scale as business strike coins, though they use the PR or PF prefix.