Originally Posted by
u600213
ORD-DCA usually starts with UA600 first flight of thee day and increments by 2 each successive flight. DCA-ORD starts with 601 and increments by 2 up to 631. But a few weeks ago there were a few flights that did not start with 6. Today they are back to all 600 series flights.
UA uses a similar convention on ORD-BOS (520s through 540s) and ORD-LGA (668 through 694), to name two.
However, I know ORD-BOS isn't as consistent as DCA (there's an 881/882 thrown into the ORD-BOS schedule), and of course ORD-LGA has the much-loved Shuttle America flights interspersed throughout the day.