Ron Guth: Collectors have identified two major varieties of 1821 Dimes: Large Date and Small Date. The 1821 Small Date Dime has an unusually small date that is atypical for the series -- it appears quite similar to the date on the 1821 Quarter Eagle, which shows the same square-base 2. The Large Date, on the other hand, features a curled base 2 in the date. The sizes of the dates are so different that an eyeball example of a single coin is all that is necessary to tell the difference.
The Large Date 1821 Dime is definitely the more common of the two varieties. As of September 2011, PCGS had certified 245 examples (in all grades) of the Large Date variety and only 60 of the Small Date variety. Mint State examples of the Large Date are scarce but not rare. The finest examples certified by PCGS are 9 MS-65s.