Keynote, Keystone. A medal associated with a series but with a feature different from all others in the series. The medal usually symbolizes the entire series and is most often struck first, or is presented with the entire purchase of the series. A Yellowstone Medal was a keystone medal for a National Park Centennial Series, 1972; or a Treasury Seal Medal with the 11-medal set of America's First Medals, 1976. The Medical Heritage Society Series issued by Franklin Mint included an Aesculapius Medal, 1969, as separate but similar item to the series. For their Apollo Flight Series Franklin Mint called the initial medal a keynote medal, as they did for a 1972 medal at the beginning of issuing the Great Olympic Moments Series. In France such a medal is called pre-series.

A keynote or keystone medal is usually issued before a series is started or shortly thereafter. An encore medal is issued after the series is completed, as if the collectors of the series are asking for a repeat performance, or another item to add to the series. Such keynote, keystone or encore medals may be larger or smaller or not even share the same specifications as other medals in the series. It must, however, bear the same theme as the series.

In cataloging, the keystone or keynote medal is usually designated "A" while those in the series are numbered consecutively in the order of their issue. An encore medal may or may not be numbered with the series.