The stopover rule in aviation refers to a break in a journey that lasts longer than 24 hours (for international flights) or 4 hours (for North American domestic flights). Unlike a "layover," which is a short pause to connect between planes, a stopover allows you to leave the airport and stay in a city for several days before continuing to your final destination. Many airlines, such as Icelandair, Emirates, and TAP Air Portugal, use "Free Stopover" programs as a marketing tool to encourage tourism in their hub cities. For example, if you fly from New York to London via Reykjavik, Icelandair allows you to stay in Iceland for up to seven days at no additional airfare cost. From a ticketing perspective, a stopover can significantly change the price of a ticket unless it is specifically permitted by the fare rules. In the world of frequent flyer miles, stopover rules are highly sought after, as they allow travelers to visit two or three destinations for the price (in miles) of a single round-trip ticket.