“Get off my mound!” the pitcher screamed, directing his rage at a base runner as he left the field following an inning-ending double-play.

He threw his glove, kicking various objects out of his way as he stomped into the dugout, his sensibilities offended by a slight very real to him, yet very puzzling to others. The pitcher, Dallas Braden of the Oakland Athletics, had become incensed earlier that inning when Alex Rodriguez of the New York Yankees had run across the mound while retreating to first base following a teammate’s foul ball.

You just don’t do that, Braden would later say in explaining what caused him to lose composure. In his mind, it’s an unwritten rule in baseball that a base runner does not walk, run, trot or crawl across his mound. That’s the pitcher’s territory, and woe to anyone who enters it. Rodriguez, for his part, expressed a mixture of puzzlement and amusement at the whole incident, saying he had never heard of such a rule. And since the incident in April, reaction across the game has mostly landed in Rodriguez’s corner.

But not entirely.

David Wells, who pitched in more than 600 games over the course of a 21-year career, told ESPN after the incident that Braden was absolutely correct to take offense at Rodriguez’s actions. But former third baseman Morgan Ensberg, who hung up his cleats in 2008 after an 8-year big-league career, bluntly disagrees.

“David is wrong,” says Ensberg, who is now a broadcaster who also blogs about baseball. “That’s like saying Braden isn’t allowed to pass through the batter’s box when he’s backing up home plate. That’s silly.”

Michael Zagaris / Getty Images During this game in April, Oakland pitcher Dallas Braden yells at Alex Rodriguez to "get off my mound." Braden later said that Rodriguez had violated an unwritten rule of baseball.

Such are the hazards of navigating a sport viewed by many as a gentleman’s game. A sport that more than any other is largely governed by a set of these so-called unwritten rules. The unwritten rules are a code of conduct, a guideline for conducting oneself on the field of play. The umpire can’t punish you if you break them, and the league can’t suspend you or dock your pay. But if your transgressions are bad enough, you will be dealt with on the field of play, for baseball players govern themselves.

Drop a bunt to break up a no-hitter? That’s a big no-no.

Hit a batter with a pitch on purpose? Expect one of your teammates to get hit.

Steal a base during a blowout? You’ll likely get a pitch aimed at your ribs the next time you come to the plate.

Try to steal the catcher’s signs? The pitch might be aimed at your head instead of your ribs.

Run across the pitcher’s mound? You’ll be yelled at — fiercely.

The problem is that the players often disagree on what exactly the rules are, interpret them differently, or choose to ignore them altogether. This leads to countless disagreements, some heated moments, and a handful of brawls every season. To understand how difficult it is to sort out, all you have to do is ask a player if he can tell when a pitcher is throwing at him. You’re likely to get a variety of answers.

MLB's Unwritten Rules Ray Stubblebine / Reuters The Code MLB is ruled by a set of "unwritten rules." But players disagree on what the rules are and interpret them differently. Full story Breaking down the most common unwritten rules   HBT Daily: Unwritten rules are stupid   Longoria: Stealing in blowout is 'rubbing it in' Vote, discuss: What do you think of unwritten rules?

“You can tell. There’s conviction behind it,” says catcher Rob Johnson, who played 61 games for the Seattle Mariners this season.

But Ensberg claims that “I don’t think the majority of the time you are (being thrown at). You definitely know if you get a ball on the far shoulder of your back. But if a guy is simply coming inside, it’s too arbitrary to really know.”

And Milwaukee Brewers All-Star outfielder Ryan Braun has yet another take: “Sometimes (you know). It depends on the situation, it depends on the game. There are a lot of things that factor in, but that’s also part of baseball.”

You can understand why conflicts arise, and the Braden-Rodriguez incident was only one of several to occur this season. In May, the Colorado Rockies caught a Philadelphia Phillies coach using binoculars in the bullpen to steal signs. In the same month, Chicago White Sox players took offense after a Florida Marlins player stole a base with the Marlins holding a 7-0 lead in the fourth inning. And in early September, Washington Nationals outfielder Nyjer Morgan started a brawl against the Marlins after he was hit by a pitch. His offense? Stealing two bases with his team trailing 11-0. And that’s just the tip of the iceberg.

Video   That's rubbing it in Oct. 4, 2010: Rays third baseman Evan Longoria thinks that stealing bases up by a lot late in the game violates baseball's code.

The rules are passed on from generation to generation. Unfortunately, they are not passed on actively — it’s not like there is an unwritten rules convention every year during spring training where rookies are taught the ways of baseball. Instead, players are expected to learn through osmosis.

“I don’t teach young guys about the rules,” admits Cincinnati Reds reliever Arthur Rhodes, who has played for seven teams over the course of a 19-year career. “They just go out and play the game hard every day and do what they’re supposed to do. If the other team hits one of our players, you might have to hit ‘em, you might not have to hit ‘em. But you don’t put all that in a young guy’s head.”

So it is assumed young players will just know what the rules are, and how to follow them.

“They’re part of the game,” claims Yankees outfielder Nick Swisher. “They’re part of the workplace, part of your job. You know what they are and you stay with them.”

But when asked how he learned the unwritten rules, Swisher cracks a grin. “When you come up you might not know every single one of them, but you’ll learn them along the way. Then again, there are probably some that I don’t know,” admits the seven-year veteran. “And hopefully by my 15th, 16th, 17th year, then maybe I’ll know more.”