Skidmore, Owings & Merrill LLP (SOM) is an American architectural, urban planning, and engineering firm. It was formed in Chicago in 1936 by Louis Skidmore and Nathaniel Owings; in 1939 they were joined by John O. Merrill. They opened their first branch in New York City, New York in 1937. SOM is one of the largest architectural firms in the world. Their primary expertise is in high-end commercial buildings, as it was SOM that led the way to the widespread use of the modern international-style or "glass box" skyscraper. They have designed several of the tallest buildings in the world, including the John Hancock Center (1969, second tallest in the world when built), Willis Tower (1973, tallest in the world for over twenty years), and Burj Khalifa (2010, current world's tallest building). SOM provides services in Architecture, Building Services/MEP Engineering, Digital Design, Graphics, Interior Design, Structural Engineering, Civil Engineering, Sustainable Design and Urban Design & Planning.