Pattern coins are often called the “coins that never were” as the designs were not accepted by the Treasury or the US Mint. For a variety of reasons, the coins were rejected such as too high relief so they can’t be stacked, too low relief so the designs don’t look proper. Many of the US Mint’s greatest and most prolific coin designers or Chief Engravers created pattern coins whose designs were not approved.
These coins were first catalogued by the US Mint and then a book was written about them by 1959. Patterns are often similar to the accepted designs, but sometimes struck in different metals.
A collection of patterns can be assembled by year, type, metals, denominations, or designers. Most have mintages of less than 1,000 coins, many well under that.