United Airlines, Continental Airlines merge to create the world's largest airline company in 2010

United Airlines and Continental Airlines merged in 2010 to form the world's largest airline company, displacing Delta.

It was only two years earlier that No. 3 Delta and No. 4 Northwest Airlines merged to leapfrog past both American Airlines and United and take the top spot among U.S. carriers, and in fact the world.

Now, in a case of déjà vu all over again, No. 3 United and No. 4 Continental imitated Delta-Northwest to become the industry leader in size. The new carrier will take United's name, once the integration is completed.

The deal closed on Oct. 1, but it'll be quite a while before the two carriers really begin to function as a single carrier. For now, United and Continental continue to fly under their own brand and operating certificates.

Continental chairman and chief executive officer Jeff Smisek has taken over CEO of the new United Continental Holdings, The new paint scheme looks like Continental's pre-merger design.

As we commented last summer, the name may be United, but it looks an awful lot like Continental.

Delta may not be the largest airline company, but it'll be the biggest airline until Continental disappears inside United. That'll take some time yet.

The implications of this deal will take some time to realize. Will the larger United be able to successfully use its mass to be more than the sum of its parts? How will Delta respond?

Also unanswered is whether American and parent AMR will feel a need to respond. But with almost all the merger partners gone, American's options remain very limited.