Yankees vet­eran right-hander Mariano Rivera, the greatest closer in the history of baseball, has made his living with basically one pitch — the cutter. (Kathy Willens/Associated Press)

The key to building a successful bullpen is to grasp that you’ll never really know what makes a good relief pitcher. Analysis of pitching styles and career trajectories is unpredictable. Every type succeeds — and fails. Some converted starters become stars, most stay bums. Freud is mostly useless, too. The crazy succeed. So do skinny guys who look like Sunday school teachers. Once you accept that you are helpless, you’re on your way.

Nobody understands relief pitchers even though everybody is searching for them. No element of the modern game is more important in an era when the final 10 outs are often divided among three to five men. Yet nobody even knows how to write a job description.

To a degree that may exceed any other position in any other sport, relief pitching is an entirely results-based job. It’s not about mastering a specific skill set or fitting a prototype. Either you can do it or you can’t.

The only way you’ll ever find out which guys have it and which ones don’t is to run ’em out there, then cover your eyes. The final score tells the tale.

“Rod Beck threw 87-88. Looked like it was down the middle. The hitters all pop up to second base and come back shaking their heads. ‘How did I miss that?’ ” said Nationals General Manager Mike Rizzo, one of the scouts who could believe what he was seeing. “Beck never changed and they couldn’t square up that pitch his whole career. Don’t ask me why.”

Beck saved 286 games, 25th best in history. Exit stage left, chuckling in his Fu Manchu. Compared to many, Jeff Reardon (367 career saves, seventh most ever) had nothing. He threw his heart on the black and dared you to hit it.

The only man to save 600 games, Trevor Hoffman, started with two years in the low minors as a shortstop and never became a top closer until he was 27. A good enough fastball and a great change-up, that’s it? Oh, and the aura that he would out-compete and beat you, no matter what, hell’s bells.

“Give me three guys with guts, I don’t care all that much about their stuff and I’ll make a bullpen around them,” Davey Johnson once said.

The reliever of your dreams could be 6 feet 6 and throw 99 mph like Goose Gossage or he could look like a walking cue stick and throw submarine like Dan Quisenberry or Kent Tekulve. Some men have thrown almost one pitch exclusively their whole career, like Mariano Rivera’s incomparable cutter, Sparky Lyle’s slider or Hoyt Wilhelm’s knuckleball.

“You throw other pitches,” I said to Lyle. “Not for a strike,” he grinned.

You don’t even know what the arc of a reliever’s career will be — even in a vague sense. They may still be bums at 25 and stars in their 30s like Todd and Doug Jones, unrelated and barely memorable, who both ended up in the top 20 in history in saves with more than 300. Command, change speeds, whatever. Who were they? How’d they do it? Who knows?

Almost as infuriating, nobody has even figured out what personality type fits the job. Renegades and extroverts — and those who aren’t bothered much by defeat because they have a hard time remembering what happened yesterday — are said to have some natural advantages. Randy Myers, ninth on the all-time saves list, kept a book, clearly visible, in the top of his locker: “101 Ways to Kill a Man With Your Bare Hands.”

But all relievers don’t call attention to themselves. They don’t scream at themselves on the mound like Al “The Mad Hungarian” Hrabosky, or advertise their eccentricity like Brian Wilson with his painted black beard or dance on the mound in their underwear in a post-World Series celebration like Jonathan Papelbon. Nobody was ever more easygoing than the No. 3 all-time saves leader, Lee Smith, a giant who could pinpoint his fastball, loved to fish, but ducked publicity completely and may, thus, avoid Cooperstown.

Who knew that, with his best stuff 10 years in the past, sidearming Dennis Eckersley could pitch to a strike zone the size of your BlackBerry. At 35 he had a 0.61 ERA, 48 saves and three unintentional walks.

So far this spring, Rizzo knows that he has assembled, by accident, a kind of ex-scout’s menagerie of classic relief pitching types. Two years ago, he inherited unadulterated trash. In a year, he had one of the better bullpens, even after he dealt Matt Capps (26 saves) at the trade deadline. Now, despite some spring relief struggles, he says, “I think we’re in a better spot now with our bullpen than we were last year.”

Drew Storen, 180 pounds, has closer-quality breaking pitches, plus a fastball that’s sufficiently alive, very much like the pitches that kept 5-foot-9 Tom “Flash” Gordon in the majors for 21 years and also served ex-Oriole Gregg “Otter”Olson well enough to get 217 saves.

Henry Rodriguez, acquired for Josh Willingham, has thrown over 100 mph and can throw the ball through a brick wall — on days when he can hit the wall at all. Sound like a young Jose Mesa or Armando Benitez? Both started in Baltimore, had their mockers (like me), got expelled from Crab City before they peaked — and ended up 13th and 24th in history in saves, combining for 610 of them and $85 million in salary. You can’t coach 100.

Tyler Clippard’s best pitch is a change-up that makes his low-90s fastball look better. “He’s all arms, legs and kneecaps. The hitters react to his 91-92 like it’s 95,” Rizzo said. Sean Burnett is another archetype — the “how does he do it” lefty who has good stuff and a freak delivery, but not, you wouldn’t think, good enough to explain a 2.61 ERA in 144 games the last two years?

The Nats are tempted to let him try closing, based entirely on his results, including a 0.00 ERA this spring; the eyeballs say his build is slight and he might be overmatched. But lefty John Franco, fourth in career saves, had results that seemed even better than his stuff. “He and Franco are comparable size and shape,” Rizzo said. Over the last five years, the lowest batting-average-against with runners in scoring position — a key stat for a reliever — is Burnett (.173). Don’t you want to find out what the ceiling is for a left-hander who looks like Stevie Ray Vaughan?

“Sean’s certainly earned a chance to do some closing,” Rizzo said.

Doug Slaten is the Southpaw Specialist who dominates left-handed hitters with sliders that look like they’ll hit the batter in the ear, but nail the outside corner instead. Even Collin Balester is a pure type: The Arm. He’s a hyper ex-surfer who burns himself up fretting when he’s in the rotation. A joyous type who brightens the clubhouse, he wasn’t meant to torment himself with “how do I set this guy up in the second inning so I can get him out the third time through the lineup.”

Tiger Manager Jim Leyland asked: “Where’s that kid with the big arm who used to be a starter for them? Have they figured out what to do with him?” No. If they send him to AAA after the spring he’s had, they’re blind.

The Nats even have a reliever headed to Syracuse — burly Cole Kimball — that Rizzo (risking sacrilege) likens for demeanor to Heath Bell.

All of this sounds hopeful. But the most important word in relief pitching is “closer.” The rest of a bullpen just sets the stage for tragedy if the wrong man has that job. The Nats haven’t picked one yet.

“Closer by committee” is often a prescription for confusion. But the Nats may have no other choice at the moment. Storen has been ragged in Florida and, at 23, they don’t want to burn him up before the team’s best talent arrives in ’12-’13. Burnett has never carried such weight. Clippard is streaky. Balester isn’t trusted in that role. Kimball isn’t ready. But some combination, perhaps Burnett and Storen, may click.

Nobody knows what makes a relief pitcher. That goes double for closers. The season starts Thursday. The Nats will have to find out if they have anybody, or two or three somebodies, who has what it takes. And they’ll find out in a hurry. Out of so many promising young arms, they probably do. But life comes at you fast. Especially in the bullpen.