ORD Boy 2:
became Chicago's primary airport in the early 60s, Terminal 1 was international arrivals and departures, while domestic carriers used Terminals 2 and 3.
Since PA had only trans-Atlantic flights out of ORD
, prior to deregulation, it's space was in Terminal 1.
When the new Terminal 1 was designed for UA
, all airlines using Terminal 1 were shifted to Terminal 4. Basically, ticketing, customs and immigration, etc. were located on the first floor of the parking garage, and large buses moved passengers to and from remote stands.
Since PA could not get any space in Terminals 2 or 3, it was stuck in Terminal 4, even though after deregulation, it had far more domestic service at ORD