The Federal Aviation Administration says a computer glitch temporarily grounded all United Airlines flights in the U.S. (Reuters)

The first thing you need to know about the United Airlines reservation-system glitch that messed up the day for about 400,000 passenger on Wednesday is: A “reservation system” is more than what you think it is. The second thing: All airline reservation systems are not pretty much the same.

All United has had to say after an almost two-hour delay that grounded 4,900 flights worldwide and caused big delays in United hubs in Chicago, Denver and Houston is: “We are recovering from a network connectivity issue this morning and restoring regular flight operations.”

The airline said a router malfunction was to blame, but it wouldn’t elaborate.

In an era when most flights are packed and tickets must be bought well in advance, stranded passengers are probably wondering why United had to ask the Federal Aviation Administration to abruptly ground all its planes over a reservation-system issue at 8 a.m. Wednesday and kept them grounded until 9:49 a.m.

The answer is that reservation systems do more than just sell tickets, and United has been having problems with its system ever since it merged with Continental almost five years ago. Several United flights also were grounded last month. It’s unknown whether those problems were also with the reservation system. The airline wouldn’t say.

Yes, reservation systems book your flight, but they also do a great deal more. Airlines use them to track maintenance schedules, for pilot and flight attendant scheduling, to create gate assignments and to manage aircraft movement to be sure there are planes at those gates.

United’s chronic problems with its reservation system are, in large measure, a consequence of the merger mania with has left the United States with four big airlines that control about 80 percent of all domestic flights.

Delta merged with Northwest, Southwest merged with AirTran, American merged with US Airways, and United merged with Continental.

With each merger came the combination of two reservation systems into one. In each case, there was a need to modify the two systems to iron out idiosyncrasies that had developed over the years, but it went more smoothly in some cases than others.

Delta and Northwest had been using the same system. American and US Airways both used a system based on the Sabre platform.

But when United and Continental merged, it was apples and oranges on the reservation-system side.

“The definition of ‘reservation system’ is an all-encompassing word. It doesn’t just mean consumers booking online or on a call center,” said Rick Seaney, chief executive of FareCompare.com. “The fact that their site wasn’t taking bookings or wasn’t giving people their award tickets correctly doesn’t stop the airline from flying.”

Were it left to him, Seaney said: “I wouldn’t call something a reservation system. I would call it a booking system, which is completely different than all these other scheduling systems they have, whether it’s pilots, flight attendants, gates, aircraft maintenance, all sorts of systems they have to move their airline.”

United had been using a system call Travelport for decades. Continental had been using a system called Shares.

“When the two airlines decided to merge, they had a choice to make: Would they take the Continental system, or would they take the United system?” Seaney said. “They decided to go with the Continental system and basically dumped Travelport.”

Flipping one reservation system into a notably different system is complicated and problematic, and it has led to headaches for United.

“There wasn’t a lot of commonality, and there were a lot of glitches along the way,” Seaney said. “They had issues with reservations, with elite status, all their awards programs had issues.”

The headaches caused by Wednesday’s glitch were visited on United’s passengers. The airline said passengers who had to change plans because of the grounding would not be charged the normal fees for such changes.

“Although the FAA has lifted the grounding ban on all United Airline flights, the travel disruption that occurred this morning will still cause a massive interruption for business travel,” said Mike Kelly, chief executive of the travel-risk-management company On Call International. “Anytime that there is an airline travel issue such as this morning’s, the trickle-down effect that it causes tends to impact business travelers and their employers for days if not weeks afterwards until operations return to normal.”

Dana Hedgpeth and Abby Phillip contributed to this report.