Collectible coins refer to various coins that are sought after by collectors for their historical significance, rarity, condition, or artistic design. In the context of the United States, the 1955 1-Cent coin, commonly known as the penny, was minted in both Philadelphia and Denver, with the Denver mint marking its coins with a "D." This particular year is notable among collectors due to various minting quirks and the presence of a famous error known as the "1955 doubled die," which significantly increases the coin's value. Collectors often seek out rolls of these cents to discover potential rarities or errors.