Collectible coins, particularly patterns, are specially struck coins that serve as test pieces for new designs or modifications to existing coins. The 1818 pattern coins from the United States exemplify this category, as they were created during a period of experimentation in American coinage. These pieces often feature unique designs and are produced in small quantities, making them highly sought after by collectors. The rarity and historical significance of pattern coins like the 1818 variations can significantly enhance their value in the numismatic market.