The "Series" date printed on US currency indicates when the note's design was adopted — It does not indicate the year a note was printed. Instead it indicates the earliest year the notes of the same design were first made. For example, Series 1882 Gold Certificates were being printed as late as 1927.
Series 1928 was the first series of modern, small-sized notes issued. Since then, the series year has been changed when there is a major design change to a bill; a minor design change is indicated by a letter suffix being added to the series year. Major design changes were rare until 1974, when William E. Simon became Secretary of the Treasury. Before 1974, a change to either the signature of the Secretary of the Treasury or the Treasurer was considered a minor change; after 1974, a new Secretary of the Treasury was considered a major change. (The only exception to this was in 1979, G. William Miller's signature appeared on Series 1977A instead of Series 1979.) A change in Treasurer is still considered a minor change. Changes in design, such as the recent changes to the $20 bill to deter counterfeiting, are still considered major changes.
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As is clear from the table, the year in the series designation does not represent the year the bill was printed. What exactly it does represent has changed over time. Originally, the series date was the year the overall bill design was authorised or adopted, and the series letter was advanced each time a minor change was made to that design (usually a change of one or both signatures). This was the rule until 1974, when William E. Simon became Secretary of the Treasury and decided that each new Secretary would result in a change of the series year, not merely the series letter. The new policy has since been followed by every succeeding Secretary except G. William Miller, whose signature appears on notes designated Series 1977A rather than the expected Series 1979.
However the series designation is determined, most changes in it are due to a change in the signatures on the notes. The following is a list of all other reasons for series changes since 1928:
 Notice that Granahan and Barr were never actually in office simultaneously, even though their signatures do appear together on the Series 1963B $1 FRN. This is the only series ever to use non-contemporary signatures this way; the exception was made only because no Treasurer was in office during Barr's very short term as Secretary.
 Note also that Elston and Kabis are actually the same person: Dorothy Andrews Elston Kabis married while in office, and changed her signature accordingly.
 A total of 90,640,000 regular and 4,320,000 star notes were printed for the Series 1953C Silver Certificate, but none were released, and almost all were destroyed. None are known to be in private hands.