The 1991 D Lincoln Memorial Cent Double Day errors are coins that were minted with the same design on both the obverse (front) and reverse (back) sides. These coins are referred to as "double day" errors because they were struck with the same design on two consecutive days. The error coins are composed of copper and were minted in the United States as part of the Lincoln Memorial series. These coins are considered circulated and are of business strike type, making them a unique and valuable addition to any coin collection.