Pilots at Delta Air Lines and Northwest Airlines may have seen the terms of their compensation and benefits change from one labour contract to the next but no figure matters more than the one next to their names on their union's seniority list.
Now, as the two airlines inch closer to a merger agreement, the pilots are grappling with how to best combine two lists into one. For some members, where they land on the list could change what they're paid, what plane they fly - even where they live.
"Seniority is really the keystone of a pilot's career," says Derek Martin, vice-chairman of the FedEx pilots' union. "Every aspect of your career, from the time you're hired to the time you retire, is driven by your seniority number. It stays with you."
Because Mr Martin was hired 12 years ago, near the start of a long recruiting wave at the delivery company, he has always reached the next phase of his career faster than many of his peers. Within two years he was first officer on a wide-bodied jet ferrying packages internationally.
The captain says the key to his rapid advancement was his position on the FedEx pilot seniority list. Its importance is also a reason Delta Air Lines and Northwest Airlines, in talks to merge since last month, may never combine.
Now that the two US carriers have found common ground on a chief executive, a corporate headquarters and a new labour contract, the deal hinges on whether the two pilots groups can agree where members will rank following the merger.
While the airlines can proceed without an agreement from pilots, its absence could undermine the companies' pledge to present a unified front with employees as they seek regulatory approval.
Delta and Northwest executives had hoped to present their boards with a definitive agreement last week, people familiar with the matter said. But as the weekend arrived the pilots were at an impasse, trading accusations of inflexibility.
Not all pilots are convinced that the combination is in their best interests.
Mike Stark, a Delta pilot who has sought to recall his union's leadership in part for supporting potential mergers, argues that whatever his colleagues gain from the new contract could be offset by a lower position on the seniority list.
"You're getting a short-term pay-off, but it's much better waiting," he said.
What is more, the further their name sits from the bottom of the seniority list, the more likely they will keep their job even as an airline take steps to slash expenses and eliminate routes.
"It's job security, pure and simple," said Charles Craver, a labour law professor at George Washington University. "You lay off in reverse order of seniority."