The gentlemen meeting in convention to ordain and establish the fundamentals of their game began with the object at the heart of things: “The ball.”

Must weigh not less than six, nor more than six and one quarter ounces … must be composed of india rubber and yarn and covered with leather. It shall be furnished by the challenging club, and become the property of the winning club as the trophy of victory.

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Section 2 concerned the bat.

And the 14-page handwritten document went on, in exquisite Spencerian script, to create what experts say are the crucial foundations of the modern game of “base ball.”

Parts of the long-lost manuscript, presented to a convention of New York-area baseball clubs in 1857 and tied with a red ribbon, will be part of a large exhibit called “Baseball Americana” that opens June 29 at the Library of Congress.

The “Laws of Base Ball,” along with two earlier drafts, have been loaned by Hayden J. Trubitt, a corporate lawyer in San Diego who said he put a mortgage on his house to buy them at auction for $3.2 million two years ago.

They were “not bought by a collector,” he said. “They were just bought by a fan.”

Lost to the public for more than a century, they are the hallowed papers of the sport, the Library of Congress said.

“There is no earlier manuscript related to the history of baseball that I’m aware of,” said John Thorn, Major League Baseball’s official historian.

He has called the articles the sport’s Magna Carta, after the famous medieval English bill of rights.

And SCP Auctions, the California sports memorabilia house that hosted the sale in 2016, called them “the most vital doctrine ever produced in the evolution of baseball.”

They sought to codify the basics of the game:

The bat must be round and not exceed two and one half inches in diameter in the thickest part. It must be made of wood and may be of any length to suit the striker.

(Current rules limit bat length to 42 inches and thickness to 2.61 inches.)

The bases must be four in number placed at equal distance from each other … upon four corners of a square, whose sides are respectively 30 yards.

The first, second and third bases shall be canvas bags painted white and filled with sand or sawdust, the home base and pitchers point to be each marked by a flat circular iron plate painted or enamelled white.

(The modern five-sided home plate and the rubber rectangle of the pitcher’s mound would come later.)

The number of players per side was established.

In playing all matches, nine players from each club shall constitute a field and they must have been regular members of the club which they represent for thirty days prior to the match.

And: The umpires in all matches shall take care that the regulations respecting the ball, bats, bases, and the pitchers position are strictly observed … [and] shall determine all differences which may occur during the game.

Versions of what would become baseball in the United States date back to at least the late 1700s, the library said. But the 1857 laws brought standardization to the game, said Susan Reyburn, curator of the library’s exhibit.

“Once you do that, then you’ve got competition that is meaningful,” Reyburn, a Dodgers fan, said in a telephone interview.

“Different teams had multiple players on the field in different numbers,” she said. “’Laws of Base Ball’ brings us nine players on the field. All of a sudden, now you can have league play. You can have teams that all agree. This is a big moment.”

The laws were adopted during a convention called to try to reach consensus on rules.

Two drafts preceded the final document presented to the convention.

One was authored by Daniel Lucius “Doc” Adams, a physician, president of the Knickerbocker ballclub, and today considered a founding father of the game. (He made balls, oversaw construction of bats and created the position of shortstop.)

The other was drawn up by William H. Grenelle, a Knicks director, Wall Street broker and one of the convention organizers, according to Thorn.

The final document, largely based on Adams’s draft, was transcribed by Grenelle, the library said.

He saved them, and they were passed down to his granddaughter, Constance Grenelle Wilcox Pignatelli.

In 1967, she wrote to the Baseball Hall of Fame, saying she had just found the laws among old family papers. She wondered what they might be worth, and whether the hall might want to buy them.

“I feel this authentic and very humanly historic record of the First Days of recorded baseball should find its place in your famous Museum,” she wrote.

But the Hall of Fame does not purchase artifacts, relying only on donated items for its collection, said Jim Gates, the hall’s library director.

Pignatelli said she couldn’t afford to donate them, and they dropped from sight for 32 years.

In 1999, they quietly resurfaced at a Sotheby’s auction in New York. They caught the eye of a collector in Texas who was not interested in sports memorabilia but thought they looked important.

“He just had a gut feeling about them,” said Dan Imler, SCP vice president. “He had just a collector’s instinct that maybe these were more important than how they were being … presented.”

“He didn’t have a full comprehension of what they were,” he said.

The buyer paid $12,650 for them, SCP said in its 2016 catalogue. Then he put them away “in a desk drawer and paid little attention to them,” the catalogue said.

But a few years ago he began to investigate, and in 2015 he asked SCP to help with the research, Imler said in a telephone interview.

The firm reached out to Thorn, and he, Imler and the Texas owner met in New York City to review the papers, he said. (The previous owner declined to be interviewed for this story.)

“My first reaction was: ‘This is the first draft of baseball history,’ ” Thorn said in a telephone interview. “No one knew they existed. … I knew I was in the presence of greatness.”

The auction was conducted in April 2016.

Trubitt, the new owner, stressed that he is not a collector.

“I don’t own any other baseball memorabilia than this, or collect anything else,” he said. “I don’t own any expensive objects.”

But he is a fan — originally of the Cardinals — and played Little League growing up in Bloomington, Ind., where his father was a college professor.

By the time he was in high school, he had memorized much of the modern baseball rule book by heart, and could recite specific passages on demand.

And in 1975 Trubitt worked as a low-ranking public relations officer with the American League, where, among other things, he fielded rules questions.

He was attracted by several aspects of the offering.

“It involved law, which I’m interested in,” Trubitt said. “And baseball, which I’m interested in, and history, which I’m interested in.”

But he did not expect to pay what he did. He said the auction house believed the items would go for a little over $1 million. But the price went way beyond that.

“The bid that won was the last bid I was going to make,” he said. “In fact, it was more than I had. I’m certainly not a poor man. But to pay for this I had to put a million-dollar mortgage on my house.”

“Part of what I was thinking was … ‘What else am I going to spend it on,’ ” he said. “It was a way to complete a circle. Baseball had been really important to me, and now in a way I sort of own baseball, or at least become a part of it, like it had been a part of me.

“There is a satisfaction in this,” he said.

But he said he has not yet held the papers.

“I’ve seen [them] in the Mylar once,” he said. But they have been in the hands of the Oregon Historical Society, in Portland, where they were first displayed, and now at the Library of Congress.

And he is not sure what he will do with them. “I can’t just keep [them] in my house,” he said. “I don’t have a moat around my house. It’d be a target.”

For now, the documents are safe in Washington, and next month when the library exhibit opens, he can come and see them, he said, “like everybody else.”