There are three major frequent flyer alliances that bring together the loyalty programs from several different airlines: Star Alliance (United, Continental, US Airways, Lufthansa, Singapore Airlines, and 21 more), One World (American Airlines, British Airways, Iberia, and nine others) and Sky Team (Delta, Air France, KLM, plus 10 more).

BBC: Insider flight tips from a seasoned business traveller

Unlike Delta's product, United's economy plus option is on both domestic and international fleets.

BBC: Flying in the fourth class

After you have torn up its 120-plus runs, warm up in the Delta's heated outdoor pool with indoor entrance, three outdoor hot tubs and a sauna.

BBC: Daily deal: Canadian mountain lodge

Regular carriers, say analysts, should offer more "in-between choices, " like Delta's Economy Comfort or United's Economy Plus, with more legroom and other amenities for a few dollars extra.

CNN: 2022: Cheap flights, more rail and hands-free cars

Delta's move follows United's recent decision to expand its popular " economy plus" seating to Continental aircraft (starting in 2012) due to the merger of the two carriers.

BBC: Flying in the fourth class