Pittsburgh is known as "The Steel City" for the extensive steel trade that fed the city's economy through the 1970s. Some folks know Pittsburgh as "The City of Bridges" because it's positioned at the confluence of the Allegheny River and Monogahela River. The Allegeheny and Monogahela Rivers combine to create the Ohio River. The three waterways create the triangular shape of downtown Pittsburgh. There are more than 450 bridges within Pittsburgh city limits. Today, Pittsburgh is home to a number of high tech companies that specialize in everything from robotics and biotechnology to internet startups. Pittsburgh is home to a number of corporate headquarters who are responsible for many downtown skyscrapers. Tied for the 8th most Fortune 500 companies, Pittsburgh is home to PNC Financial, U.S. Steel, H.J. Heinz Company, WESCO Internations, Mylan Labs, CONSOL Energy, PPG Industries who supply paint and optical coatings, as well as Dick's Sorting Goods who operate over 408 national retail locations. Pittsburgh is a major manufacturing city but that doesn't mean they don't have world class entertainment and culture available. The Andy Warhol Museum, Carnegie Museum of Art, and the ArtGardens are just a few options that locals have. The Pittsburgh Symphony Orchestra calls Heinz Hall home. Heinz Hall also hosts a number of other concerts like Bruce Springsteen and other natural disaster aid concerts. The rap song "Black and Yellow" by Pittsburgh's Wiz Khalifa is one example of great local musicians. George Benson, Perry Como and Christina Aguilera all hail from Pittsburgh or Pitts suburbs. No wonder Pittsburgh is ranked consistently as "one of the most livable places in the United States."