Southwest Airlines, Inc. based in Dallas, Texas, is a low-fare airline in the United States. It is the third-largest airline in the world by number of passengers carried and the largest in the United States by number of passengers carried domestically. Southwest was originally incorporated to serve three cities in Texas as Air Southwest on March 15, 1967, by Rollin King and Herb Kelleher. Some of the incumbent airlines of the time (Braniff, Trans-Texas, and Continental Airlines) initiated legal action, and thus began a 3 year legal battle to keep Air Southwest on the ground. Air Southwest eventually prevailed in the United States Supreme Court, which ultimately upheld Air Southwest's right to fly in Texas. December 7, 1970, the date of the Supreme Court decision, is considered by many to be the de facto beginning of deregulation in the airline industry.