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 even would have 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) who hung onto 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’s 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.”

Dave Dombrowski, Red Sox president of baseball operations

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 539S 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.