Mergers often produce changes in airline fleets. In some cases, they result in the introduction of a new type into the acquiring/surviving carrier's inventory, sometimes with significantly positive results. Such was the case when Delta acquired Northeast in 1972, and with it, added the 727 to the DL stable. Of the U.S. trunk carriers extant at this time, only Delta had eschewed the Boeing "three-holer". Northeast, while of quite modest scope compared with its new partner, had partaken of both the 727-100 and -200, and was the launch customer, as well as first operator, for the latter type.
Delta had chosen to rely on the smaller Douglas DC-9 as its jet replacement for piston aircraft on short/medium-haul domestic routes. Reflecting its long relationship with Douglas, the Atlanta-based carrier was the launch customer for the DC-9, placing the initial order in April, 1963, "no doubt resulting from a friendly chat between Donald Douglas Sr. and C.E. Woolman" (Delta's founder and long-time CEO), according to the late R.E.G. Davies' Delta: An Airline and Its Aircraft.
Second only to KLM, Delta had operated almost all of the "DC" piston aircraft types from the DC-2 to the DC-7, and, for that matter, was the initial operator of the DC-8 jet. This continued past the DC-9 in the jet era with the DC-10 (twice, briefly) and the MD-11, which finished out Douglas (by then McDonnell-Douglas) commercial production. (KLM had pride of place at Douglas due to being the only customer for the DC-5, and the Delta DC-2s were acquired from American Airlines, rather than the manufacturer. The DC-1 was essentially a prototype, built to the specifications of, and initially operated by Transcontinental and Western Air, known later, and more familiarly as TWA.)
The initial Delta DC-9s, -14 models, entered service in 1965. The inaugural aircraft carried the title "Delta Prince", although its later siblings were nameless. Why the lesser-royalty appellation is not known, but transportation aficionados know that the "Delta Queen", and its sister ship, the "Delta King" (somehow that doesn't sound right), were sternwheel steamboats, after all. The -14s were supplemented by a much larger group of -32s, beginning in 1967; they served until January 1, 1993. Apparently many had become enamored enough of Atlanta's Hartsfield Airport that they simply changed ownership (and colors), becoming ValuJet "Critters", and continued serving a number of the routes that they had plied while in Delta's employ. (The joke about this was that navigation at ValuJet was easy; the airplanes already knew the way.)
While Delta continued to utilize many aircraft operating on the DC-9 type certificate, in the form of the MD-88, the DC-9-30 had disappeared from the roster. However, the type re-appeared in October 2008 when Delta merged with Northwest. NW also had become, subsequent to its own acquisition of Republic, a significant operator of DC-9s, of several models; some had originally flown with Delta, including a pair of -32s. (Interestingly, seven of the -14s, among them the very first Delta aircraft, had gone first to Southern Airways and then Republic before finding themselves in the Northwest fleet, but these had been retired by the time of the DL/NW amalgamation.)
Their reprise at Delta was brief, with the -30s (and similarly-sized -40s) retired in 2010, although the DC-9-50s remained, and continue to soldier on. However, it looks like -- at least from a visual perspective -- the DC-9-30 will be returning to Delta in the not distant future. Delta has arranged to take the Boeing 717 fleet that Southwest acquired via merging with AirTran (itself the descendent of ValuJet). Yes, I know that the 717 (which began life as the MD-95, prior to yet another merger: Boeing/McDonnell Douglas) is not a truly a DC-9, but it certainly gives the appearance of one, albeit with up-to-date engines, and an MD-80-style tail even if it is a Boeing product. And if it looks like a DC-9, and flies like a DC-9...what else could it be? After all, it's on the same FAA Type Certificate (A6WE, Revision 28) as all the DC-9 and MD-80 models, and for that matter, the MD-90, which Delta also has been adding to the fleet of late. (No, better halt here; this has become confusing enough already.)
Baseball great Yogi Berra's famous "déjà vu all over again" statement barely begins to tell the tale...or is it "tail" in this case?