1.   But if a traveler chooses three frequently visited city pairs, Expedia will E-mail the lowest fares each week.

2.   In the United States, regional city pairs like those in the Northeast corridor approximate that kind of geography.

3.   Not surprisingly, fares were lower in city pairs that had competitive low-fare service, the department said.

4.   The analysis also showed huge variations among airlines serving the same city pairs.

5.   As carriers increase their non-stop city pairs and flight frequencies to cater to customer preferences, the average size of planes in airline fleets is declining.