Carter Kieboom is ranked the 16th-best prospect in baseball by Baseball Prospectus. (Jonathan Newton / The Washington Post)

Here is how someone realizes they can throw a baseball with both hands: They spend the first seven or so years of their life throwing as a righty, one day have a bat in their right hand so pick up a ball with their left hand instead, and then, upon tossing it, realize their form and power are just as sound.

That’s all it takes. At least for one kid who grew up to be a professional.

“I was a batboy for my older brothers’ teams and when my I couldn’t use my right hand to pick up a ball, I just used my left hand,” said Carter Kieboom, the Washington Nationals’ top prospect, in mid-January. “And it felt the exact same when I threw.”

The 21-year-old Kieboom is already down in West Palm Beach, Fla. for his first major league spring training, even with most Nationals position players reporting to camp on Monday. He will look to impress as the league’s 16th-best prospect, according to Baseball Prospectus, showcase his defensive ability at shortstop and second base, and find the offensive rhythm that made him shine across two minor league levels last summer. But his most unusual skill won’t be seen by coaches, teammates or fans in the coming weeks.

He hasn’t consistently thrown with both hands since he was both a right- and left-handed pitcher for Walton High School’s junior varsity team as a freshman. Yet ask those who watched Kieboom’s rise when they realized he was different, a budding star, the kind of player who could thrive in baseball-crazed Georgia and later become a first-round pick, and many say this tipped them off.

“He’s a stud, and he’s also kind of a freak,” said Spencer Kieboom, Carter’s older brother who is a 27-year-old catcher in the Nationals organization. “He can throw with both hands, for starters. I mean who does that?”

Not many people. Kieboom isn’t quite ambidextrous — which would mean he could do everything with both hands — but he is some odd version of it. He hits right-handed, writes with his right hand, shoots a basketball with his left hand, shoots pool with his left hand and throws a football left-handed. One mid-January morning, as a handful of players gathered for a strength-and-conditioning workout, Kieboom lobbed passes to Colorado Rockies center fielder Charlie Blackmon inside Rapid Sports Performance in Woodstock, Ga. Each one hit a streaking Blackmon in stride. Kieboom’s left hand released each ball without a hitch.

One old family photo, of Kieboom as a 9-year-old, shows him pitching right-handed for the East Cobb Stallions. But whenever he and Spencer played in the backyard, Carter stepped onto a makeshift mound and fired in pitches as a lefty. He always had more fun that way. That led him to use both hands as a JV pitcher, wearing a six-finger Mizuno glove (with two thumb holes) that allowed him to change hands without leaving the field.

He stopped using both hands after he moved up to varsity as a sophomore, as he then spent most of his time as a shortstop and third baseman. But he could still throw a low-80s, left-handed fastball as he got older, and James Beavers would occasionally utilize that. Beavers coached Kieboom with the East Cobb Yankees, an elite travel team for 16-, 17- and 18-year-olds. If Beavers needed to get out a lefty in a key, late-inning situation, he’d ask Kieboom to move from third base to the pitcher’s mound to get the job done. And Kieboom almost always did.

As for why this didn’t translate into Kieboom becoming a switch hitter, a very attractive skill if developed properly, Beavers never saw the two skills connecting.

“I think that’s just a different tool altogether,” Beavers said. “But it was funny and it made you pause. He’s very talented. He could probably pick up a glove today and throw left-handed and you’d look at him and wouldn’t think twice about him being a lefty."

Kieboom’s spring training will be filled with other objectives. He will be the second-youngest infielder working alongside established players like Anthony Rendon, Brian Dozier and Trea Turner, among others. He will keep learning to play second base so he can possibly take over for Dozier in 2020, as the veteran signed a one-year, $9 million deal in January. And he will look to stand out while blending in, a bright prospect who wants to meet expectations while also paying his dues.

But if the Nationals did ever need a player to throw with both hands, even if there is no situation in which that would be the case, they have that rare ability in the organization.

“Oh, I could definitely still do it,” Carter said while laughing. “I don’t think that’s something you lose.”

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