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
Willis Tower the law firms of Schiff Hardin and Seyfarth Shaw. Morgan Stanley plans to move to the building in 2019, and to become the building's fourth-largest tenant by 2020. In 1969, Sears, Roebuck & Co. was the largest retailer in the world, with about 350,000 employees. Sears executives decided to consolidate the thousands of employees in offices distributed throughout the Chicago area into one building on the western edge of Chicago's Loop. Sears asked its outside counsel, Arnstein, Gluck, Weitzenfeld & Minow (now known as Arnstein & Lehr, LLP) to suggest a location. The firm consulted with local and federal authorities