Once upon a time, a baseball team constructed its pitching staff the way your contractor constructs your hallway -- from the front to the back. Boy, life in this sport sure was a lot less complicated then.

You didn't just need starters. You needed great starters. You needed innings from your starters. You needed horses. You needed men who could take a lead and hand it to the closer. Or to the eighth-inning guy.

Remember when we thought that was the only way to get through a season? The only way to win a World Series? The only way to analyze who could win the World Series?

Well, this just in: We don't live in that universe anymore.

The team that just won the World Series -- your Kansas City Royals -- finished 24th in the major leagues in starting-pitcher ERA last season. And 26th in innings eaten by starting pitchers. So how many other champs in the expansion era ever ended up that close to the bottom in both categories? Only one -- the 1976 Big Red Machine. Led by a manager (Sparky Anderson) known as "Captain Hook."

But this just in too: It isn't only the champs who are hammering away as hard on the back of their house as the front. Are you familiar with the New York Yankees' work lately?

They led the American League in save percentage last year. They already had a two-headed bullpen monster, the much-feared Dellin/Andrew/Betances/Miller-zilla beast. Then they did something last month that no team ever would have even thought about doing five or 10 or 20 years ago: They added a third fire-breather to the monster pod, in Aroldis Chapman.

Amazing, right? Except that, wait a minute, if you look around, you'll find something else incredible about 2016 bullpen construction:

We count seven teams (including the Yankees) that hung on to their incumbent closer this winter -- but still added a reliever who either led his old team in saves last year or finished the season as that team's closer. Those seven teams: Yankees, Boston Red Sox, Houston Astros, Toronto Blue Jays, Texas Rangers, Oakland Athletics and (how about this?) even the Royals, who brought back Joakim Soria.

Now there's no rule in our handy dandy Columnists' Guide to Chronicling Huge Baseball Developments that tells us when something has officially become a Trend. But even if loading up on bullpen arms isn't a trend yet, it has definitely become a Thing. Don't you think?

"It's been the wave. Let's face it," said Boston's president of baseball operations, Dave Dombrowski, a man who waited like 15 minutes into his offseason to pull off a stunning trade for Craig Kimbrel. "A lot of clubs have improved the depth in their bullpen. And it has paid off."

So what's going on? Why has this happened? How has this happened? And how is it changing life as we used to know it? Keep reading. You're about to find out.

It's the Royals

Like any team that showers in October ticker tape, the Royals are aware they've started something that other teams would love to copy. But the funny thing is, as much as they've enjoyed unleashing Greg Holland, Wade Davis and Kelvin Herrera on poor hitters everywhere, even their front office isn't certain how (A) innovative or (B) intentional its own formula actually is -- or was.

"It's been the wave. Let's face it. A lot of clubs have improved the depth in their bullpen. And it has paid off."

It was just two weeks ago, in fact, that the team's assistant general manager for player personnel, J.J. Picollo, was kicking back, watching a documentary on the legendary "Nasty Boys" bullpen brigade of the 1990 Cincinnati Reds, when he had the distinct feeling that, hmm, something looked familiar.

"I thought: We didn't invent this. It was done 25 years ago," Picollo said. "And we just stumbled upon it."

Yes, it's true. It was never the Royals' 2015 master plan to build a staff in which the relievers devoured 539⅓ innings, no matter how unhittable those innings may have turned out to be. It just happened -- because they wound up with a rotation that didn't digest enough innings.

"It is a blueprint," Picollo said. "But really, the way it evolved for us was more out of necessity."

So here's their message to teams thinking about copying their formula: They'd prefer not to even have to copy it themselves. Not this year -- or any future year.

"We don't think it's a recipe for success," Picollo said, "to get less innings from our starters, by any stretch of the imagination. ... It can be done. But it's not the way we want to do it. We can't expect our bullpen to pick up that type of load over what's really a seven-month season. It's just too much to ask."

Hey, good point, actually. Want to guess how many other teams have ever had their bullpens throw that many regular-season innings on the way to winning a World Series? Not a one. Of course.

It's the starters

But it isn't only the Royals who found the upside in hooking their starters earlier than ever. Now that, we're hereby ruling, has turned into an official trend.

Most starts under 6 IP Year Starts 2015 2,003 2009 1,987 2006 1,978

According to baseball-reference.com, on an astounding 2,003 occasions last season, starting pitchers failed to make it through the sixth inning. That's the most in the history of baseball.

Not surprisingly, a bunch of terrible teams led the sport in that category. But right behind them were six teams that contended for most of the season -- the Tampa Bay Rays, Yankees, Royals, Rangers, Cubs and Minnesota Twins. And in some of those cases, that wasn't just how it worked out. It was a reflection of data that clearly shows it's a bad idea to let many starters go through a lineup a third time.

Here's a chart that compares relievers working through a lineup the first time with starters facing a lineup the third time in 2015. It tells you all you need to know.

AVG OBP SLUG OPS K PCT Relievers .244 .315 .389 .699 22.27 Starters .278 .334 .417 .764 17.62

So why would a manager intentionally create matchups in which the average hitter's OPS goes up by 65 points, while his chances of striking out decrease by more than 25 percent? More and more managers are asking themselves that question -- and gonging starters earlier than ever.

When Joe Maddon was in Tampa Bay, the Rays were way out in front of most of the sport in avoiding third-time-through disaster. So Maddon carried that philosophy to Chicago with him last year. The result? In more than a third of the games the Cubs played -- 57, to be precise -- Maddon's starters went no more than five innings. That was the most ever for a Cubs team with a winning record.

"But you can't do that with every one of your starters," Maddon said. "Or else you'll kill your bullpen."

So Maddon managed Jake Arrieta and Jon Lester dramatically differently than how he managed the rest of his rotation. Those two combined for only eight of those abbreviated starts. On the other hand, Jason Hammel and Kyle Hendricks combined for 28, all by design.

"I think you always want to build around your rotation," Maddon said. "You always want to find four or five guys who you dig, who can give you innings and keep you in games. But you don't always have that. ... So everyone's different. You just don't throw everyone in one bucket and say the third time through, everyone loses effectiveness. When Jake Arrieta is on the mound, that's just not true."

But how many Jake Arrietas roam this planet? Not enough. So the info age has let managers and GMs everywhere see that when the mere mortals are starting, bullpen depth has never mattered more than it does in this moment in baseball time.

It's the data

As we were saying, it isn't just a lack of elite starters that's producing Bullpen Mania. It's the numbers. It's the data that's driving more pitching changes, more quick hooks and more bullpen-matchup moves than at any time in recorded baseball history.

Most relievers used in a season Year Relievers 2015 15,095 2012 14,514 2014 14,454

There were 15,095 pitching changes made last season. That's the most ever, over 600 more than the year before and about 2,000 more than a decade ago. And it's about 4,000 more than we were witnessing two decades ago. In a related development, 2015 was also the first season in history in which the average team went to its bullpen more than 500 times.

"We're all more matchup-oriented," Maddon said. "And that's just based on information. It's based on information that's indicating you really should change pitchers right now."

If you have a rotation like Maddon's, in which two of the five starters save the bullpen on days they pitch, you can manage ultra-aggressively on the other three days. So he did. And he had plenty of co-conspirators. Between the matchup data and the third-time-through data on starters, why wouldn't a guy manage that way?

But roster construction has to adapt in lockstep with how those games are managed. And teams are getting that memo. In the Cubs' case, they look to fill out the staff with bullpen candidates who were former starters and who are what Maddon describes as "matchup-neutral." Meaning they can get out hitters from either side and pitch multiple innings in the middle of games.

Average relief appearances per team Year Relief appearances 2015 504 2005 439 1996 404 1985 281 1975 225 (* 1995 was a strike-shortened season)

So once they shifted Travis Wood to the bullpen last season, and converted reclamation projects Trevor Cahill and Clayton Richard to valuable relief roles, "that really shifted the tide for us," the manager said. "We won so many games last year where we were a run or two down in the middle of a game and we were able to bring somebody in to keep the game right where it was. We didn't think we were ever out of a game."

Meanwhile, the Yankees took an equally innovative, but very different, approach to eating those innings last year. They bounced so many pitchers back and forth between the Bronx and their Triple-A team in Scranton, they wound up using 26 different relievers, the most in franchise history. But that Scranton shuffle was possible only because they built a staff filled with minor league options and roster flexibility. And it worked.

"We mapped out that strategy going into the season," said GM Brian Cashman. "We knew our rotation wasn't going to go into the seventh inning with any certainty. So we had to find a way to fill multiples of very important innings. ... And that added up to a lot of innings. Whether they were productive innings didn't matter. They were productive in the sense that somebody needed to log those innings."

Once, teams used 26 relievers in a season only out of desperation. Now it's out of modern roster-building creativity. The transaction column may never be the same.

It's the money

You know all that stuff we just talked about? It was interesting and fun and all that. But it's always about the money. You know that. I know that. And face it: The cost of acquiring relief pitchers, even great ones, is puny compared with the ever-exploding cost of top-of-the-line aces these days.

It's a simple math equation, when you get right down to it. The highest-paid relief pitcher in the major leagues is Jonathan Papelbon. His last contract guaranteed him $12.5 million a year (although he'll earn only $11 million this season, as part of a restructured option).

Meanwhile, what does a decent starter cost? We're now up to seven active starting pitchers who will earn at least double that $12.5 million a year, on average, over the life of their current contracts. Zack Greinke ($34.4 million a year) will almost triple that.

And a total of 24 different starters have contracts paying them an average of at least $12.5 million a year. That group includes Rick Porcello, John Danks and even Edwin Jackson. So clearly, they're not all aces. Get the picture?

"It has to do with [this]: You're putting money into your team; where are you going to put it?" Dombrowski said. "From a financial perspective, you can get bullpen guys at a less expensive price than you can with your starting rotation."

Not that that stopped Dombrowski from committing $217 million to David Price's E*Trade account. Because the man who runs the Red Sox still believes that no matter how good your bullpen is, you need an ace to win.

Still, the cost of acquiring relievers is almost always right, relatively speaking, whether you're signing them or trading for them. And that cost factor shows up in all sorts of ways, on all sorts of levels. Not just when teams are constructing their big league rosters, but even when they're crafting their draft strategies.

The Royals, for instance, go into almost every draft looking for pitchers they think have a shot at ace-hood. And that's not simply because you can never have enough aces hanging around your system. It's because of -- what else? -- long-term economics.

"Our goal is to take starters because we can afford [big league] relievers," Picollo said. "We can't plug in [free-agent] starters because we can't pay those guys. So economics certainly factor into it, especially for a small- to mid-market team."

It's the health factor

Here's our general rule of thumb: Pitchers get hurt. They all get hurt. Or it seems that way. But it turns out that's not completely true. Bet you didn't know that the average big league starter is much more likely to end up on the disabled list than the average reliever. That's a fact.

The Dodgers' former vice president of medical services, Stan Conte, recently formed a company called Conte Injury Analytics, which has analyzed injuries incurred during the 15-season period from 1998 through 2012. Over that span, 50.3 percent of all starting pitchers landed on the disabled list per year. The rate for relievers, on the other hand, was just 32.6 percent.

That study doesn't factor in the extent of those injuries, or usage factors, or distinguish between pitchers who are on the roster all season versus pitchers who are just filling a roster spot for a few days. So it's not a perfect measuring stick. Conte readily concedes that. Nevertheless, he said, "There is little doubt that starters get injured more than relievers."

So there you go. Just one more reason this sport is engulfed in Bullpen Mania.

It's the future (or maybe not)

We haven't even gotten into the rapidly growing inventory of bullpen creatures who can propel a baseball 95, or 98, or 103 miles per hour. This is also fueling Bullpen Mania. But regardless, Cashman said, "it has always been easier to build great bullpens than great rotations."

"The value of 200 innings pitched is so much greater, but it's hard to find," he said. "Starters are harder to find than relievers. It's pretty much that simple."

"It has always been easier to build great bullpens than great rotations ... Believe me, I'd rather have five guys who can give you nine innings every day and give you 250 innings a year, and have an ERA under 2.00, and compete for Cy Youngs. But I don't have that."

So you might think that when the Yankees traded for Aroldis Chapman instead of, say, signing Price or Greinke, they were being trendy. After all, we've just spent 2,000 words documenting that building crazy, dominating bullpens is what the smart teams do these days.

But to think that or suggest that, said Cashman, would be "100 percent" incorrect.

"We spent all winter trying to explore various opportunities in free agency or the trade marketplace, including a variety of starting pitchers," he said. "And I have Chapman -- and Starlin Castro -- to show for it. . . . So you get what you get, and you don't get upset. That's what I tell my kids."

In other words, for the record, constructing a pitching staff from back of the game to front is not now an official Yankees philosophy. It was simply a matter of practicality, given all the other moving parts and a price tag, for ace starters, they weren't interested in paying.

"Believe me, I'd rather have five guys who can give you nine innings every day," Cashman said, "and give you 250 innings a year, and have an ERA under 2.00, and compete for Cy Youngs. But I don't have that."

Instead, he's locked into a rotation with health and innings issues -- "so the only way to counteract that," he said, "is to have very strong presence in the bullpen."

Here, then, is what we've figured out while working on this opus. The way-too-easy conclusion is to look at Bullpen Mania and decide that teams are just trying to copy the formula of the team that won the World Series. But here's the reality: Good luck with that.

"Do we all try to learn from the team that won the World Series? I don't think there's any question," Dombrowski said. "But it's hard to be in a spot where you copy what they did. ... You can say 'OK, we're going to copy them.' But the truth is, you've got to get the right guys. It's not that easy to duplicate."

And no one understands that better than the Royals themselves. In 2014, they had three relievers with ERAs under 1.50. By the end of 2015, their closer, Greg Holland, was heading for his friendly neighborhood Tommy John surgeon and wasn't even on their postseason roster.

So in 2016, they've "replaced" him by signing Soria. They still love their bullpen. And if people want to emulate them, they're flattered. But as much as the game has changed, they don't look at themselves as a team that has just thrashed out a revolutionary new template for 29 other teams to follow.

They view themselves as a team that has shown that if you can't win a World Series the old-fashioned way, well, at least there's an alternative path out there that just might work.

"I still believe most teams want to do it through the rotation," J.J. Picollo said. "But you know what? If you don't have the horses, at least now you know there's another way it can be done."