Delta, Continental, Northwest in Alliance

Delta Air Lines Inc., the nation’s third-largest airline, plans to join Continental Airlines Inc. and Northwest Airlines Corp. in a broad new partnership aimed at protecting revenue from a proposed rival alliance.

The airlines plan to sell seats on one another’s flights and coordinate schedules and frequent-flier benefits under a “code-share” agreement. The term comes from the practice of putting an airline’s two-letter industry code onto another’s flights. The proposal must be approved by federal regulators and pilots’ unions.

Atlanta-based Delta, Houston-based Continental and Northwest, based in Eagan, Minn., hope to have the agreement in place by the spring.

The partnership comes in response to a similar relationship between United Airlines and US Airways that was announced last month.

Delta shares fell $1.03 to $18.97 on the NYSE. Continental fell 21 cents to $10.80, also on the NYSE. Northwest fell 55 cents to $10.90 on Nasdaq.