In 1866, the United States Mint in Philadelphia produced trial coins and patterns for the ten-dollar gold piece, known as the "Double Eagle." These pattern coins are important collectibles as they represent designs that were considered but ultimately not adopted for circulation. The coins can feature various design elements, including modifications to the obverse and reverse, making them sought after by numismatists. The rarity and historical significance of these trial coins contribute to their value in the collectible market.