The scene was Laurel, the Greene Turtle sports bar, on a mostly deserted Tuesday night. There was a $1.50 special on draft beer, a TV screen as big as a barn door -- and an invisible baseball barrier running between the stools.
"It's right here," said Bernie Berry, pointing out the imaginary line separating him from friends Charlie Gallagher and Jim McElwaine.
Berry supports the Baltimore Orioles; he has for decades. "A fan at 5, a fan at 50," said Berry, 36, meaning he's committed for at least 14 more years.
But Gallagher and McElwaine have gone over to the other side, becoming supporters of the Washington Nationals.
"That's the home team," said McElwaine, 41, of Olney. "What's wrong with rooting for the home team?"
The same line is being drawn across Maryland, especially in counties where Washington's suburbs bleed into Baltimore's. In what used to be hard-core Orioles territory, the appearance of the Nationals has divided families, workplaces and Little League teams.
The volume has been raised in recent days, now that the Nationals and the Orioles are in first place in their divisions.
It's not a blood feud yet -- just a healthy and sometimes cranky rivalry. But underlying the divide is what drives apart fan bases: disputes over history and loyalty, and different conceptions of where home is.
"There's definitely true, hard-line alliances already," said Tim Stevenson, a baseball coach in Severna Park who has seen his adolescent players debating which team is better: the Nats or the O's.
In Cameron Rahnama's family, where he supports Baltimore and his brother-in-law Washington, the teams' winning ways mean both sides have more ammunition.
"We went out to dinner the other night -- he's ranting and raving about how the Nationals are in first place," said Rahnama, the athletic director at Howard's Glenelg High School. "And I'm like, 'The Orioles are in first place, too.' "
It is hard to tell how many people have changed allegiance, because neither team's front office could provide statistics about fans in the border areas.