Nowadays if you are an airline operating within the US domestic market and you wish to serve a route pair you simply do just that, after relatively little fuss and bother, but prior to 1978, when the US market was regulated by the Civil Aeronautics Bureau (CAB), starting new routes was a marathon process. Indeed, most of the time getting permission to start a new route, especially if there was any competition involved, was a non-starter. Whence the regulated period threw up a selection of idiosyncratic practices that made sense then but look a bit weird today. One of these was the concept of interchange services.
I'm Richard Stretton: a fan of classic airliners and airlines who enjoys exploring their history through my collection of die-cast airliners. If you enjoy the site please donate whatever you can to help keep it running: