The master hub is a steel bar with the coin's design on the end in raised relief just like the coin. It is used to create the working dies which are used to strike the coins.
A master hub is created from a Janvier transfer reducing machine
that traces a larger than actual coin design from a galvano or epoxy shell. The resulting design on the hub is the actual size of the coin.
Die Varieties vs Hub Varieties
Master hubs don't always have all the details needed to make the working dies. Things like the date or mint mark may be stamped directly into the working dies before they are ready to be used.
Differences in these dies are called die varieties and collectors frequently collect coins by their die varieties. Since a single master hub can be used to make many dies, you usually don't hear about hub varieties, but they do exist.
One of the more interesting hub varieties
is that of the Double Eagle gold coins. From 1850 until 1866, Double Eagle dies were created from only two master hubs. The first was used from 1850 until 1858. The second from 1859 until 1866.
What makes this interesting is that the first master hub actually had the word LIBERTY misspelled as LLBERTY. This was corrected by placing an I over the second L, but the original mistake can be clearly seen on those Double Eagles struck from 1850-1858.
If you are lucky enough to have a Double Eagle gold coin from 1858 or earlier, get out your magnifying glass and take a look.