Up until 1933, gold coins circulated freely.  The United States Treasury minted gold coins in the following denominations
to be used in everyday commerce: $1, $2.50, $3, $5, $10 and $20.  Numerous "pattern" coins were minted in gold of which
the $4 piece is probably the best known and recognized.

Although there are six denominations, a complete type set of US gold coins will contain a total of 15 coins due to the
fact that there were three different designs of the $1 piece, and the $2.50, $5, $10 and $20 each had two different designs.
Add in the $3 "Princess" and the total number is 15.  Assembling a US gold type set is a popular goal among collectors
and investors.