I noticed this quote in the latest Delta Digest, talking about Northwest's fleet:

"Domestically, Northwest's smaller-sized narrowbody aircraft (DC-9s and Airbus 319s) fill a gap in Delta's existing mainline fleet," said Nat Pieper, vice-president-Fleet Strategy. "Delta's primary domestic aircraft are MD-80s and 757s. On routes where demand may exceed a regional jet's capacity, but not enough to use an MD-80, a DC-9 or A319 will offer a more optimal quanity of seats to better meet the demand," Nat said.