The United States, Australia, and to a lesser extent Brazil, Mexico, India, the United Kingdom, and Japan have "deregulated" their airlines. In the past, these governments dictated airfares, route networks, and other operational requirements for each airline. Since deregulation, airlines have been largely free to negotiate their own operating arrangements with different airports, enter and exit routes easily, and to levy airfares and supply flights according to market demand. The entry barriers for new airlines are lower in a deregulated market, and so the U.S. has seen hundreds of airlines start up (sometimes for only a brief operating period). This has produced far greater competition than before deregulation in most markets. The added competition, together with pricing freedom, means that new entrants often take market share with highly reduced rates that, to a limited degree, full service airlines must match. This is a major constraint on profitability for established carriers, which tend to have a higher cost base.
Moreover, the industry is structured so that airlines often act as tax collectors. Airline fuel is untaxed because of a series of treaties existing between countries. Ticket prices include a number of fees, taxes and surcharges beyond the control of airlines. Airlines are also responsible for enforcing government regulations. If airlines carry passengers without proper documentation on an international flight, they are responsible for returning them back to the original country.

Puerto Rico is a commonwealth of the U.S., so citizens do not need a passport to visit this island. This is a bustling island full of activities. Cities such as San Juan and Ponce offer lots of historical sites as well as great food and nightlife. There are lots of different kinds of beaches, making it equally attractive for surfers, snorkelers or people who simply want to relax. Puerto Rico also has quite a few casinos both large and small.

Nicknamed “The Helen of the West” (an allusion to the beauty of Helen of Troy), St. Lucia stuns with its signature feature: the UNESCO-listed twin Pitons. Reaching heights of about 2,500 feet, the voluptuous volcanic spires complement the island’s other attractions, including verdant jungles, sparkling silver-sand beaches, haunting sugar-estate ruins, and a mineral-rich natural mud bath. Meanwhile, the island’s most famous resort, Jade Mountain, is an architectural gem in its own right. 

Since airline reservation requests are often made by city-pair (such as "show me flights from Chicago to Düsseldorf"), an airline that can codeshare with another airline for a variety of routes might be able to be listed as indeed offering a Chicago–Düsseldorf flight. The passenger is advised however, that airline no. 1 operates the flight from say Chicago to Amsterdam, and airline no. 2 operates the continuing flight (on a different airplane, sometimes from another terminal) to Düsseldorf. Thus the primary rationale for code sharing is to expand one's service offerings in city-pair terms to increase sales.
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