"Everybody in New York knows what they're going through," said Mike Petti, a 48-year-old nicknamed Yankee Mike who for 13 seasons has been a staple of the bleacher section where the most hardcore fans those who hate Red Sox Nation the most dwell. "When it happened here, Boston was singing 'New York, New York.'"
The teams' rivalry, which has reared its ugly head in bench-clearing brawls and fan assaults over the years, is just one difference between the cities, both among the oldest in the country and each with a rich history.
But look around New York this week, and you'll see nothing but love for Boston.
In what city officials said was a first, the bright-blue Boston city flag flew at half-staff at New York City Hall, on the orders of Mayor Michael Bloomberg, who grew up in the Boston suburb of Medford.
"We stand shoulder to shoulder with our friends in Boston, as all Americans do," said Bloomberg, who in 2006 founded the group Mayors Against Illegal Guns with Boston Mayor Thomas Menino. Bloomberg said he phoned his counterpart Monday to offer him any help New York could provide.
An Occupy Wall Street group, the Illuminator, on Monday night projected the two teams' logos in large lights on the walls of the Brooklyn Academy of Music. A heart was inserted between the logos, to read "NY(heart)B." The image has gone viral.
New York Roman Catholic Cardinal Timothy Dolan said a special prayer for Boston on Tuesday during his Mass at St. Patrick's Cathedral.
"You've got our love, you've got our hope and you've got our solidarity," he said later. "You're going to get through it."
Joe Daniels, CEO of the National September 11 Memorial and Museum, sent out an email offering comfort to Bostonians.
"The 9/11 Memorial is a constant reminder not only of what we have endured as a result of terrorism, but also of our ability to come together with limitless compassion," he wrote. "In the wake of the Boston attacks, this spirit of unity is more important than ever."
On Comedy Central's "The Daily Show," host Jon Stewart touched on the rivalry in his opening monologue Tuesday night.
"Oftentimes the two cities are accusing each other of various levels of suckitude," he said. "But it is in situations like this that we realize it is clearly a sibling rivalry, and that we are your brothers and sisters."
Tension has bubbled between Boston and New York since the 17th century, when the Puritans, who founded Boston, and the Dutch, who founded New York, squabbled over Long Island.
"We're so small compared to New York, but we're so powerful," said Northeastern University historian Bill Fowler, acknowledging that Boston, with just over 600,000 residents, is smaller than the borough of Brooklyn. "On per capita basis, we've got you beat, it's just that you're bigger."
Yankees fan Steve Sanzillo, is still going back and forth with his wife, a Sox fan, over which baseball team their 19-month-old son, Jackson, will root for. For him, Monday's attack did more to unify the cities than any rivalry could divide.
"It just brings back all the memories of 2001," he said. "I think I speak for all New Yorkers by saying our hearts are with Boston."
Diamond's "Sweet Caroline" makes has no specific mention of Boston or the Red Sox, but the team started playing it regularly at Fenway Park more than a decade ago and fans took to it, singing along as loudly as possible.
Ballparks across the country played the song during Tuesday's games, but the sing-along was especially poignant at Yankee Stadium, where many fans are residents of a city no stranger to terrorism.
"I've been getting a lot of looks, kind nods," said Boston native Christine Sanzillo, 34, wearing a Red Sox hat alongside her husband, a New York native. "It's a positive energy here today. Once my mother had a beer poured on her head."
During a moment of silence at the start of the game, a large commemorative ribbon with logos from the two teams was shown on the electronic board atop the ballpark.
"But what's going to happen when the Knicks and Celtics square off?" Petti Yankee Mike said, referring to the upcoming playoff game between the two cities' basketball teams.
"They'll show some respect," he said, "then they're going to want to beat each other."