The most famous collaboration between Hugh Stubbins and LeMessurier, this landmark skyscraper was originally known as Citigroup Center and was the world’s 9th tallest upon completion. Over 900 feet in height and extremely efficient in its structural design, several structural innovations were employed in this tower: four mid-side super columns at the base allow the corners of the tower to cantilever over the city block below; a pair of super diagonals every 15 stories on each face of the building direct the gravity and lateral loads to the main columns; and the world’s first Tuned Mass Damper controls building motion under wind loads. This tower’s legacy continues to be the ingenuity of design and its peerless execution.