Here's how I like to break down the subtle differences between the three hubs in terms of service. PHL is the primary international gateway for US with about 9 daily transatlantic flights(PIT has 3 to LGW, FRA, CDG and CLT has 2 to FRA, LGW). CLT stands apart from the the others with it's extensive Caribbean service. CLT also has flights to most medium to big sized cities within the US Airways system. PIT is the big feeder hub for small towns (Johnstown, Latrobe, Elmira, Ithaka, Erie, etc...) because of the number of daily express flights they have--far more than either of the other two. PIT also has the largest number of transcontinentals. Pre 9/11 PIT usually had more daily departures over CLT simply because of the express flights but CLT as far as I know has always had more mainline flights. Sine 9/11 it's been pretty close. One month PIT will have more departures, the next it will be CLT.