Series (United States currency)
On U.S. currency, the series refers to the year appearing on the front of a bill, indicating when the bill's design was adopted. The printed series year does not indicate the year a bill was printed; instead it indicates the earliest year that bills of the same design were first made. For example, Series of 1882 gold certificates were being printed as late as 1927.
The first U.S. currency with a series year was printed on United States Notes introduced in 1869. Before that, paper currency was identified only by the act authorizing it, for example, the act of March 3rd, 1863. For these bills, the serial number uniquely identified the bill, except for some issues that exceeded one million bills. In that case, the sequence of serial numbers was restarted, and an extra overprint of 'Series 1' was added to the bill. When one million bills in 'Series 1' were printed, 'Series 2' was used, and so on. 'Series 187' is the highest series number of this sort that was used, on the United States Notes of 1863, in the $5 denomination.
Other notes were identified by a date of issue. Interest-bearing notes were carefully identified as to the date of issue, because they reached maturity a fixed amount of time later. Gold Certificates, issued upon the government receiving a deposit of gold, were dated by hand, and also the depositor was identified. Only the depositor could redeem an early gold certificate. Starting with the Series of 1882, Gold Certificates were made payable to the bearer.
National Bank Notes were given a date as well. However, this was not when the bill was first circulated, but rather when the bill was sent from the federal government to the issuing bank, which could then release it at its own convenience. National currency, issued and unissued, has a series date (corresponding to when the design was last changed) and a release date.
The first series printed by the Federal Reserve was Series 1914. It contained a $5 bill with Abraham Lincoln, a $10 bill with Andrew Jackson, and a $20 bill with Grover Cleveland, all of which were large-sized notes.
The next series was Series 1918, which had large-sized notes in denominations from $50 to $10,000. The portraits were the same as the ones mentioned in United States dollar with one exception; the $1000 bill had Alexander Hamilton.
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.