The condition is that you have to be connecting with that carrier to or from an international flight. But many airlines offer stopover discounts to all comers. Emirates Airline offers special fares and stopover deals in Dubai to passengers traveling between Europe and Asia. If you are traveling through Hong Kong, watch for similar stopover deals on Cathay Pacific Airways. Major travel agents such as American Express, Carlson Wagonlit Travel and Rosenbluth International negotiate volume discounts with carriers in the form of ''net fares'' to sell on to individual travelers.
Some corporations demand it.
Another device is ''combination tickets,'' whereby you combine two separate trips on one ticket. The fare from New York to Johannesburg is exactly the same as the fare from London to Johannesburg, and you would logically fly through London. So effectively, you travel the Atlantic free. You don't have to specify dates in advance.