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.