Double dies are the result of a die that's been struck more than once by a hub in misaligned positions, creating doubled design elements on the die itself. Strike or machine doubled examples are easily confused with genuine double dies. Their doubling only exists on the coin, the result of being struck more than once by a normal die in an abnormal manner.
The 1873 DDOS1 is referred to as the "King" of the series due to its spectacular doubling on a desirable type coin, the closed 3. The key marker is the separation of all letters of LIBERTY, but the doubling of the feather stems, eye, lips and ear are also obvious to the naked eye.