Southwest Airlines, the largest airline in the U.S. domestic market, in terms of passengers, has obviated a long-held truism with the recent addition of 737-800s to its fleet. (Beware: reading further will require summoning up your inner airline geek.) For many, many years, the carrier could be depended on, absolutely, to operate aircraft with only a single overwing emergency exit on each side of the aircraft. The -800, of course, has a pair of overwing exits, so this lengthy tradition is being abolished. It appears, based on the May 22 announcement of a tentative agreement between Southwest and Delta, that Southwest could operate some former AirTran 717s. These aircraft also have a pair of overwing exit rows, but in any case, the 737-800 has entered the Southwest fleet first, since AirTran is still being operated separately at this point.
When Southwest began service in 1971, it utilized what was one of the smaller commercial jet aircraft types, in the form of the Boeing 737-200. This was the Seattle manufacturer's "entry-level" jet, in terms of seat capacity at the time, since deliveries of the smaller -100 model had been completed. Eschewing what would be the "older/used/cheaper" paradigm of the coming breed of new entrants in the late 1970s and into the 1980s, Southwest chose to go with new from the start. (In fact, as the Texas carrier went into business, there were few role models to look to for advice on these matters. California-based Pacific Southwest (PSA) did start with "cheap cast-offs", in the form of DC-4s, but moved up into new equipment with the Lockheed turboprop Electra, and by the time of Southwest's inaugural flight, had new widebodied equipment on order, in the form of Lockheed L1011s.)
Like PSA, Southwest also expanded outside its original intrastate operating territory once U.S. passenger airline deregulation occurred, beginning in 1978. The Dallas-headquartered airline moved up to slightly larger equipment when they, along with US Air became the launch customers for what would become known as the "Classic" series of the 737, specifically the -300. While the 737-300 offered a modest fuselage stretch versus the -200, it still retained a single exit row on each side over the wings, however.
As Southwest grew further, it dabbled in the 737-500, which had the same fuselage length as the -200, and therefore, the same seating capacity. In the mid-1990s, this time sans USAir, Southwest launched the next generation of the 737, the -700, which had the same seating capacity as the -300, albeit with an improved wing and engines that produced range and speed advantages. Boeing later produced two further "stretches" of the 737, in the form of the Dash 800 and 900, and even retained the -200 capacity via the 737-600; Southwest demurred on this one, however.
As time progressed, Southwest and its now merger partner, AirTran, remained the stalwarts of the 737-700, while other carriers opted increasingly for the higher-capacity -800 and -900. And now, Southwest has effectively ended the -700 era, by choosing the larger model for its route system going forward (although there will be plenty of -700 capacity in its fleet for years to come). And thus, Southwest moves up to an aircraft with a pair of over-wing exit rows.
The historians among you will object, however, because this isn't the first time. On two occasions, in 1979-early 1980, and again in the early-to-mid 1980s, WN operated 727-200s, a type that definitely featured two overwing exit rows; there is a legitimate reason if the carrier is experiencing a deja vu moment or two on this score. (The 1979/80 iteration was a one-off, in the form of a Braniff aircraft provided as part of an anti-trust settlement; this was deemed to provide too much capacity, especially outside the morning/evening peaks, and was difficult to turn using the airline's extremely short station dwell times. Between 1983 and 1985, WN leased a small group of 727-200s from PeoplExpress, and again concluded that their system could be run more efficiently with a single aircraft type of somewhat smaller capacity.)
To put this in broader perspective, it's useful to examine the average number of seats in the worldwide jet fleet, via a look at the independent aircraft forecast published in the respected Airline Monitor annually in the journal's July issue. In 1970, according to the Monitor, the average aircraft had 136 seats. By 1980, this had grown to 165, and then to 178 as of 1990. Largely as the result of the widespread introduction of regional jets, the figures for 2000 (177) and 2010 (176) actually showed modest declines. However, by 2020, the forecast is for 187 seats per aircraft, with growth to 196 by 2030. Southwest, long a bellwether in airline profitability, apparently has concluded that slightly bigger is going to be better, suggesting that those betting on the larger 737 and A320 family members, as well as the Boeing 787 and Airbus A350 may very well be on track going forward.