You might recall the single famous quote that summarizes a big difference between Southwest Airlines and United Airlines

But while that inspiring quote tells the story from a passenger's perspective, there's an important difference between the two airlines from a business perspective, too.

In fact, that difference comes down to a single four-letter word--one that contains within it an important lesson for any business. No, it's not that four-letter word. (Or that one.)

Instead, the difference between Southwest and United that I'm thinking of comes down to "KISS." As in, "Keep It Simple Stupid."

The acronym has its roots in the military, and I'm pretty certain I first I heard it in English 101 at Fairfield University, taught by the late Ann Spector, almost [censored] years ago. (My wife and I met in Ann's class, and I remember it all quite well.)

By the way, the "stupid" at the end of the quote isn't really meant to be insulting; it's just a nod to how even the brightest among us can complicate things that should be easy, by adding too many variables.

And that brings us to Southwest and United--and a simple thing Southwest does that hasn't really translated at United. Although, as we'll see, perhaps that's changing.

The Boeing 737.

Let's briefly cite a couple of similarities between the two airlines that play into this, and which will make the the glaring difference more obvious.

Southwest and United are the No. 3 and No. 4 United States airlines according to passengers flown. It's pretty close--just under 158 million passengers for Southwest; just over 148 million for United. 

Their fleets are almost exact the same size: 754 planes for United; 731 for Southwest.

But that similarity belies the big difference. Because for most of its history, Southwest has flown really only one kind of plane: variants of the Boeing 737. The exception to this was when Southwest leased some Boeing 727s from other airlines; but that practice ended more than 30 years ago.

Flying the same planes cuts down on maintenance and employee costs. It also means that literally every single one of Southwest's pilots is rated to fly any one of its jets.

That's not the case at United, which flies six different types of airplanes by my count, along with multiple variants of each one. And that doesn't include the multiple types flown by the contracted carriers that operate as United Express.

Understanding the cost of complexity.

I was prompted to think of this important distinction after United's earnings call this week. 

Actually, it was Justin Bachman at Bloomberg who zeroed in on it in the call, focusing on how United's acting CFO, Gerry Laderman, said the airline was "getting much better at really understanding the cost of complexity of operating multiple fleet types."

Bachman cited this in the context of the hurdles that some airplane manufacturers may now face in getting United to consider even more new types of planes.

And as United president Scott Kirby said on the same call, sometimes you just can't profitably add yet another kind of plane to the fleet, because employee salaries and operating costs outpace revenue:

"[You] can do the math. You take an airplane that's nicely profitable and you turn it unprofitable with that kind of cost structure. And so it's purely about economics."

Cocktail-napkin business plans.

Southwest's spirit of simplicity comes through even in the airline's apocryphal origin story, which is that its two co-founders supposedly sketched the original idea for the airline in a bar, on a cocktail napkin.

It's not really fair to compare that to United's story. It's about twice as old an organization, includes multiple classes of service, has been through at least as many different corporate organizations, including mergers and bankruptcies, and flies a much more complex worldwide series of routes and destinations. For instance, you couldn't cover United's 8,700 mile, 17-hour, Los Angeles to Singapore service with a medium-range 737. 

Yet it's precisely when an airline--or any business for that matters--becomes so complex that the KISS principle can become most important. Complexity begets more complexity--at least until somebody with a little power can step back and observe with a bit of perspective.

It seems United has been doing better on the customer perception front lately. And in the hours after the earnings call we're talking about, its stock price jumped about 8 percent. 

So maybe this focus on simplicity explains why businesspeople and investors perceive some improvement as well. 

By the way, I asked United if they had any comment on this whole simplicity issue beyond what was said in the earnings call, but I haven't head back. We'll update this if they  provide any other information. 

Simple enough.