General Doubling question

Millertime

Would someone please explain machine doubling and hub doubling? I'm guess a double die variety isn't cause by these methods so would someonle also explain how a double die variety is caused?

Thanks,
Millertime

relayer

double(d) die
A die that has been struck more than once by a hub in misaligned positions, resulting in doubling of design elements. Before the introduction of hubbing, the individual elements of a coin's design were either engraved or punched into the die, so any doubling was limited to a specific element. With hubbed dies, multiple impressions are needed from the hub to make a single die with adequate detail. When shifting occurs in the alignment between the hub and the die, the die ends up with some of its features doubled – then imparts this doubling to every coin it strikes. The coins struck from such dies are called doubled-die errors – the most famous being the 1955 Doubled Die Lincoln cent. PCGS uses doubled die as the designation.

double-struck
A condition that results when a coin is not ejected from the dies and is struck a second time. Such a coin is said to be double-struck. Triple-struck coins and other multiple strikings also are known. Proofs are usually double-struck on purpose in order to sharpen their details; this is sometimes visible under magnification.

RVDavis

Here is a doubled (actually trippled) star on an 1878 Morgan.