The certificate from the Federal Aviation Administration clears the two operations to use a single carrier code and combine operations, culminating more than a year of paperwork and reviews. For passengers, it enables Delta to present itself and function fully as a single airline. Among employees, pilots from both airlines will be able to fly together and other workers can begin to join forces...

On Jan. 1, the “invisible curtain between the two certificates” in flight control, aircraft routing and crew tracking will disappear, he said...

Northwest’s Web site, nwa.com, will disappear. Delta and Northwest pilots will be trained on each other’s aircraft and will be able to share a single cockpit, and the airline will gain full flexibility to schedule planes and routes from the two carriers interchangeably...

Flight attendants and ground workers will still work under different rules and job terms until representation elections are complete.