From the record
“Nauvoo Historic District is a National Historic Landmark District containing the city of Nauvoo, Illinois. The historic district is nearly coterminous with the City of Nauvoo as it was incorporated in 1840, but it also includes the Pioneer Saints Cemetery, the oldest Mormon cemetery in the area, which is outside the town boundary.”
Wikipedia article Nauvoo Historic District, CC BY-SA 4.0.
One of 9 National Register listings published here for Hancock County · listed in 1966 · category: District · a National Historic Landmark.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service

