This rivalry is real, perpetual, and, mostly, downright nasty.
Evolving from state lines and bloodlines - and it's mostly bad blood - it's been one of the league's most heated since the teams were born in 1967. Now, for the first time, it will decide a Stanley Cup finalist as the Pennsylvania teams meet in the Eastern Conference finals beginning Friday night.
"All the games (during the season) were heated and now we go into the conference finals and I'm expecting it to be heated again, and even more," Flyers defenseman Kimmo Timonen said.
Only Timonen won't be part of it. He is expected to miss the series with a blood clot in his left ankle that developed during the second-round series against Montreal, a major setback for the Flyers.
Timonen is their most skilled defenseman and was expected to be matched against Pittsburgh leading scorer Evgeni Malkin's line.
"We have to view this that he's not a player for us in the series, and march on," Flyers general manager Paul Holmgren said.
Mimicking their locations in diametric corners of one of the East Coast's biggest states, the teams are polar opposites, which makes the talented Timonen's absence all the more significant.
The Penguins, much like the days when Mario Lemieux and Jaromir Jagr were the team's big names, are flash and dash, speed and flair with scoring stars Malkin, Sidney Crosby and Marian Hossa. They've won eight of nine in the playoffs and all five home games, a relatively easy start for a youthful team whose three biggest stars, including goalie Marc-Andre Fleury, are 23 or younger.
The Flyers, while possessing multiple scoring lines like Pittsburgh, are rightful descendants of the franchise's fabled Broad Street Bullies, winning through toughness and intimidation. They muscled up to the Penguins' stars, pushed them around, distracted them with some success while winning five of eight during the season.
A season series that, on Dec. 11, required only 20 seconds for the first fight to start.
Asked when the rivalry might kick in again, Penguins forward Ryan Malone said, "Probably right when the puck drops (Friday night), I think."
Don't think there's a difference between the style of play in the usually free-flowing Western Conference and the rivalry-filled East? Consider this.
In their four regular-season games, Western Conference finalists Dallas and Detroit combined for 26 power plays and two power-play goals. In their eight games, the Flyers and Penguins combined for a remarkable 86 power plays and 20 power-play goals.
"I think the only difference in us that you'll see from the regular season is that no one wants to take that extra one," Penguins defenseman Rob Scuderi said. "It's OK to be physical, it's OK to play the body, it's OK to play hard, but no one wants to take that extra penalty."
Not with Philadelphia No. 3 and Pittsburgh No. 4 in playoff power-play conversions, with the Penguins getting at least one power-play goal in all but one game.
If there's a common theme between Flyers coach John Stevens and Penguins coach Michel Therrien - who, coincidentally, faced each other twice in AHL playoff series - is that it's as important for their teams to stay under control as it is to control the other.
"We want to be aggressive. We want to play with urgency and intensity," Stevens said. "But if we're undisciplined, we're just neutralizing ourselves."
For the Penguins, that means finding a way to slow Pittsburgh native R.J. Umberger, who had eight goals in the second round against Montreal and six during the season against them.
For the Flyers, that means somehow containing the multiple waves of Penguins scorers that will emerge from the Malkin and Crosby lines to challenge goalie Martin Biron.
"We know they will target guys like Crosby and Malkin and Hossa and (Petr) Sykora, but that's fine," Therrien said. "That's the playoffs. Ottawa tried to do it. The Rangers tried to do it."
That they're facing each other to play for the Stanley Cup is something new. That each has figured in the other winning Cups is not.
In 1969, the Penguins passed on defenseman Bobby Clarke in the draft at No. 15 and the Flyers grabbed the future Hall of Famer at No. 17. In 1990, the Flyers picked Mike Ricci at No. 3, allowing Jagr to fall to Pittsburgh at No. 5. The Penguins won the Cup for the first time 11 months later, then captured it again a season later after the Flyers dealt them forward Rick Tocchet and defenseman Kjell Samuelsson.
Maybe there is some brotherly love in this rivalry after all. Then again, maybe not.
During the Penguins' 10-7 win in Game 5 of the 1989 conference semifinals, angry Flyers goalie Ron Hextall chased forward Rob Brown across the ice after Brown's goal made it 9-2. Equally embarrassed and incensed, the Flyers won the next two games and the series.
Crosby's introduction to the rivalry came by the blunt end of Flyers defenseman Darian Hatcher's stick in November 2005, leaving Crosby with some chipped teeth, a bloody mouth and an unsportsmanlike conduct penalty for protesting the lack of a penalty. No doubt that's one reason he has 37 points in 20 career games against them.
Now, there could be seven Flyers-Penguins games over two weeks in what truly will be a heated state of hockey.
"You know, why not beat each other up a little bit to get on to the next round?" Malone said, laughing.
Gloves and visors for the fans are optional.
"They're notorious for being physical and being on the edge of dirty, and that's the way you have to play in the playoffs," Penguins defenseman Hal Gill said. "But you have to toe that line and be smart. That's what the challenge is going to be."