Aon Center (Chicago) the world headquarters of Aon and Amoco. Aon's US operations are still headquartered here. The building is also the co-headquarters of Kraft Heinz. The Standard Oil Building was constructed as the new headquarters of the Standard Oil Company of Indiana, which had previously been housed at South Michigan Avenue and East 9th Street. When it was completed in 1973, it was the tallest building in Chicago and the fourth-tallest in the world, earning it the nickname "Big Stan". (A year later, the Sears Tower took the title as Chicago's and world's tallest.) The building employs a tubular steel-framed structural system
Metropolitan Tower (Chicago) White, the Metropolitan Tower was named the Straus Building when completed in 1924. Though it was the first building in Chicago with 30 or more floors, it was never officially designated Chicago's tallest building since the Chicago Temple Building, also completed in 1924, is taller by but has seven fewer floors. The Straus Building and the Chicago Temple Building were the first to take advantage of the 1923 zoning ordinance; before then, no building in Chicago could be taller than . The Metropolitan Tower was at one time called the Continental National Insurance Company Building (later Continental Center I). From