Major League Baseball’s free agency is a contractual concept that is just about as easy to understand as Einstein’s theory of relativity.

But one bit of this complex player/owner relationship that most fans understand is the following: “A player with six or more years of major league service who is not under contract for the following season is eligible to file for free agency.”

In other words, he can sign with any other club that will have him if he so desires.

The idea that a player could put his talent up for bids on an open market came about in December of 1975 after two pitchers, Andy Messersmith of the Los Angeles Dodgers and Dave McNally of the Montreal Expos, played the 1975 season without signing contracts, claiming that their old contracts could not be renewed if they were never signed. David Seitz, an arbitrator, agreed with them.

Messersmith and McNally won a major victory, Dec 23, 1975, by becoming the first MLB players in history to be declared “free agents.”

Thus ended the longstanding MLB reserve clause which bound a player to the original team he signed with as long as that team wanted to keep him, regardless of salary or conditions of servitude. If traded to another team under the reserve clause, the new team then had the player for as long as it wanted him.

Free agency has led to ever-increasing salaries for MLB players, since they can negotiate with more than one team to sell their talents. It has also led to considerable disaffection by some fans who do not like the idea of players jumping from one team to another regularly. These fans claim they can’t root for their old team because it never remains the same from year to year.

Well, they are partly right and partly wrong.

Not once since 1903 when the Boston Red Sox beat the Pittsburgh Pirates in the first World Series has the reigning champion remained exactly the same for the following season. All other MLB teams changed personnel even more than the championship team.

Now, with players able to move of their own choice, teams look different from season to season, just the way they did over a century ago. Nothing has really changed, except some fans have the incorrect belief that the reserve clause kept teams intact while free agency tore teams asunder.

Contrary to that misinterpretation, three outstanding players have set a MLB togetherness record as they are now in their 16th season as New York Yankee teammates. No other trio of players in MLB history has remained together on one team for this long.

The three Yankees are Derek Jeter, shortstop and team captain, Mariano Rivera, possibly the finest closing pitcher in MLB history, and Jorge Posada, the Yanks’ catcher. Jeter and Rivera seem sure bets for the Baseball Hall of Fame, while Posada may have an outside chance for the Hall.

Bob Waterman, of the Elias Sports Bureau in New York City, told me that not only has no other trio of players been together on one team for this long in more than a century of MLB, but no three athletes have played together on the same team for this long in any of the other major American professional sports leagues — the National Football League, the National Hockey League or the National Basketball Association.

Jeter, who is 35, Posada at 38 and Rivera at 40 all joined the Yanks in 1995 and earned their fifth World Series ring last fall.

I came across a couple of other MLB threesomes that played together on one team for more than a decade. Of course, one was the most famous baseball trio in history — the Chicago Cubs’ Joe Tinker at shortstop, Johnny Evers at second base and Frank Chance at first base.

This trio gave birth to the celebrated double play refrain of “Tinker to Evers to Chance” that echoed loud and clear for 11 seasons, 1902 through 1912. And they shared in two World Series championships, 1907 and 1908.

Another Yankee trio of Yogi Berra, Mickey Mantle and Whitey Ford played together for 13 seasons, 1951-1963. Mantle and Ford, who were very close friends on and off the diamond, played together for 17 seasons as Yankees, 1951-1967.

John Smoltz and Tom Glavine, two future Hall of Fame pitchers, were Atlanta Braves teammates for 15 seasons, 1988-2002. Greg Maddux made it a pitching threesome for 10 of those years, 1993-2002.

Some people think Babe Ruth and Lou Gehrig were together forever. But in fact, these two original sluggers who gave the Yankees their moniker of Bronx Bombers played together for 12 years, 1923-1934.

Two other long-lasting but troubled teammates were the Detroit Tigers’ outfielders, Ty Cobb and Sam Crawford. They played side by side for 13 years with Cobb in center and Crawford in right field. They were also two of the best hitters in the American League over those years, 1905-1917.

Cobb came up in 1905 as a rookie two years after Crawford was sold to the Tigers by the Cincinnati Reds. At the start, these two outfielders had a good relationship. But this soured, and by the time Crawford retired after the 1917 season, the two were very unfriendly Tigers. The nasty, cantankerous Cobb had few friends, while Crawford was very popular with other team members and with the public.

Fortunately for the Yankees, Mantle and Ford were close buddies, and Jeter, Rivera and Posada happen to be three very likeable gents.

There are 204 former MLB players in the Baseball Hall of Fame. Forty-six of them or 22.5 percent played their entire major league careers with only one team.

Among those one-team stars are a few such as Robin Yount of the Milwaukee Brewers, Tony Gwynn of the San Diego Padres, Cal Ripken of the Baltimore Orioles and Mike Schmidt of the Philadelphia Phillies, who played part or all of their one-team careers during the free agency era. Yet they never were tempted to jump ship, despite multi-million dollar offers to move.

Fans get jumpy when their favorite players are approaching the season of free agency. These most passionate devotees of a given team are afraid one of their heroes will be hitting home runs or pitching no-hitters against their team instead of for their team come next season.

Gordon White served 43 years as a sports reporter for The New York Times. His e-mail is sports@thepilot.com