The graph displays the other subjects mentioned on the same pages as the subject A series of citadels on Sullivan's Island, South Carolina, built to protect the city of Charleston. If the same subject occurs a page with A series of citadels on Sullivan's Island, South Carolina, built to protect the city of Charleston more than once, it appears closer to A series of citadels on Sullivan's Island, South Carolina, built to protect the city of Charleston on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.