Harbaugh once recruited Carroll's son to the University of San Diego. (Carroll was not present for the home visit.) Stanford beat USC as a heavy underdog in 2007, but USC claimed revenge the next season. The most memorable interaction came on Nov. 14, 2009, when they met for the customary postgame handshake after Stanford's 55-21 victory. Harbaugh had called for a two-point conversion late in the second half, which was a dose of gamesmanship in a humiliation of USC.
"What's your deal?" Carroll said as the two shook hands.
When Carroll was asked Thursday what the deal is now in his relationship with Harbaugh, Carroll could not help but snicker.
"For whatever reason, [reporters] have had a field day thinking it's something other than it is," Carroll said. "He's a tremendous football coach. . . . That's where it starts and stops."
Harbaugh won two of three meetings between their teams in college and four of six in the NFL. But the Seahawks won the NFC West this year, and their last two wins over the 49ers came in Seattle by a combined score of 71-16.
When asked about animosity between the coaches, Harbaugh told reporters it was "erroneous." Carroll credited Harbaugh's general approach and philosophy with turning around Stanford and the 49ers.
The Eagles will play the Seahawks and Niners next season, and Kelly adds another wrinkle. The former Pac-10 coaches coached three of the NFC's six playoff teams. Kelly has former players on both the Seahawks and 49ers and a relationship with both coaches. The Eagles coach complimented them in his introductory news conference.
Seahawks cornerback Walter Thurmond and center Max Unger were both at Oregon while Kelly was there. Both say they believed that Kelly would make similarly successful gains in the NFL.
The success of all three could help erase the stigma of NFL teams hiring coaches from the college ranks. The difference between Kelly and Carroll and Harbaugh is that the latter two worked previously in the NFL, while Kelly had never spent a day in the league until Eagles owner Jeffrey Lurie offered him the job.
"There is always a question about that: Can a college guy coach in the NFL?" Carroll said this week. "If a guy has had an NFL background, then I think it makes an enormous difference. . . . Hopefully, if we've added to that, then it's great for the opportunity for those guys coming out of college ball."
Thurmond said Carroll is doing the same thing with the Seahawks that he did at USC: stockpiling talent and fostering competition. Only four players on Seattle's active roster predate Carroll. Two of his top contributors - cornerback Richard Sherman and wide receiver Doug Baldwin - played for Harbaugh at Stanford.
Sherman and Baldwin noted the differences between the two coaches. They said Harbaugh is more of a disciplinarian, while Carroll's approach with the Seahawks promotes personality. Sherman, who is one of the NFL's most colorful players, revealed how he would answer questions under Harbaugh:
Question: Do you think you guys are going to win?
Answer: I think it's going to be a tough battle.
Carroll has developed a popular persona in Seattle - on the field, in the locker room, and in front of cameras.
"He's not soft, but he's easygoing," Sherman said of Carroll, whom he called the "polar opposite" of the reputation of NFL coaches. "He allows his players to be who they are within the confines of the team. As long as it doesn't hurt the team, he allows guys to be themselves."
The Seahawks-49ers rivalry transcends the coaches. Sherman said the success of the teams is the reason. The narrative would be different if both franchises were 4-12.
"There is no love lost; there is no love found," Sherman said. "I don't know if there are going to be handshakes after this one."
As history has shown, even those handshakes can be entertaining when Carroll and Harbaugh are involved.