Even back when hubs were the 'end-all be-all' of air travel in the US, two airlines seemed to be about all the big cities could handle.
Frontier, United and Continental all hubbed at Denver Stapleton in the 1980s (even TWA gave it a go between 1978 and the early 1980s.) UA
were the more successful... Frontier never really got the critical mass it needed to be more than just a feeder for the big guys. Passengers could take Frontier from smaller cities (Grand Junction, Casper, Omaha, Billings) and connect to United or Continental flights to the big cities. Most of the travel agencies in Denver were loyal to United, which didn't help either. Ultimately, Frontier collapsed, and most of its fleet and Stapleton gates were absorbed by Continental.
Braniff, Delta and American had a similar go-around at Dallas/Fort Worth in the late 1970s and early 1980s. Braniff was the big dog at DFW
before Deregulation, but American and Delta had more of the lucrative east-west routes that make DFW
such an excellent hub. Braniff's DFW
routes were mostly north-south, except for a few flights to New York, Washington and the Pacific Northwest. Although BN
basically had an entire terminal and one whole side of the airport to itself, they expanded too rapidly and couldn't make the hub concept work. American came out the victor, and Delta quickly snapped up the extra capacity when BN
went under. The 'new' Braniff initially tried hubbing at DFW
, but eventually retreated to the less-threatening Kansas City in the mid-1980s.
Those are the big three-airline hubs I always think of. Some cities have served as major focus points for several airlines (Miami was a big base for Eastern, National and Pan Am, for instance, and Delta had a sizeable presence there; United, American, TWA and Continental all had decent-sized ops at LAX
as well, but nothing I'd consider a hub.) Two airlines seems to be the magic number for a hub - although today ORD
are the only true two-carrier hubs in the US.