Manchester City and Manchester United face off for the 188th time this weekend, yet for respective managers Pep Guardiola and Erik ten Hag the contest at the Etihad Stadium will be their first ever meeting.
That is not to say the pair are unacquainted with one another, though. In fact, that could not be further from the truth, with the two men sharing very similar philosophies and having previously worked closely at Bayern Munich.
While Ten Hag may be a year older than his counterpart at 52, the Dutchman was the apprentice and Guardiola the master during that overlapping period in Bavaria between 2013 and 2015.
"That time was enormously valuable for me. I remember it well," Ten Hag told Bild in 2018. "Many people in Holland shook their heads when I left the Eredivisie for the Regionalliga."
No wonder. Ten Hag was a burgeoning head coach after guiding Go Ahead Eagles to their first promotion in 17 years in his only season at the club.
The decision to then take charge of Bayern Munich II, the reserve side of a Bayern team that were enjoying a period of sustained success under Guardiola at the time, was a big surprise.
For Ten Hag, the sideways move – at best – was considered an opportunity to learn from Guardiola and a better path for his long-term career. The past decade suggests that was the correct call.
Ten Hag went on to manage Ajax for four-and-a-half years following success with fellow Eredivisie club Utrecht, before being handpicked by United as the man to bring trophies back to Old Trafford – or so they hope.
"I never regretted the decision," Ten Hag recalled in that interview with the German outlet. "Working for a club as big as Bayern with such formative personalities as Pep Guardiola and also Matthias Sammer was like winning the lottery."
Ten Hag's possession-based style – though adaptable as we have come to see in his first two months at United – has drawn natural comparisons to compatriot Johan Cruyff, a man Guardiola himself took a great deal of inspiration from.
Rather than the 'Total Football' that was developed and famed by Cruyff, though, Ten Hag looked to do things the 'Guardiola Way' across his four full seasons with Ajax, during which time he won three Eredivisie titles and took the Dutch giants within seconds of a Champions League final.
"His philosophy is sensational," Ten Hag said of Guardiola in 2019. "What he did in Barcelona, Bayern and now with Manchester City, that attacking and attractive style sees him win a lot. It's this structure I've tried to implement with Ajax."
Implement it he did, and so impressed were United they not only went all out to land the Dutchman, despite it being apparent that Mauricio Pochettino would be available, they allowed him to also have a huge say in transfer activity ahead of his first season at the helm.
Of the five senior players signed, two joined directly from Ajax (Lisandro Martinez and Antony), one had previously played for the Eredivisie side and came through their youth ranks (Christian Eriksen), and another had spent his entire career to date playing in the Dutch top flight (Tyrell Malacia).
With that level of turnover both on the field and in the dugout, patience was always likely to be required among United fans. Two games in, the Red Devils found themselves bottom of the division after losing by an aggregate 6-1 scoreline against Brighton and Hove Albion and Brentford.
Not so much 'Total Football', but 'Total Rubbish' in the view of many. While not as extreme, Ten Hag also made slow starts at previous clubs, including first side Go Ahead Eagles when drawing two and losing one of his first three games, and also at Utrecht with a record of two wins, two draws and two defeats.
With four wins from four since the Brentford debacle, it is fair to say Ten Hag has slowly won supporters over ahead of his first taste of the Manchester derby. Not that it has come as any surprise to Guardiola, the man seeking a fifth Premier League title in the past six seasons with City.
"He is a top-class manager and his teams are a joy to watch. Watch his Ajax team and you see his qualities," Guardiola said upon confirmation of Ten Hag's appointment at United in April.
"To define a manager you just have to take a look at his teams for a long time and this is a team where their manager makes them play. There's no doubt about that. But the relationship we had was incredibly good."
In an alternate universe, Ten Hag would have spent another season or two in the Dutch capital before succeeding Guardiola in the Etihad Stadium dugout. Indeed, Guardiola said as much earlier this year.
"Are you asking me if Erik ten Hag could be here? Definitely," Guardiola said at a news conference when asked if the then-Ajax coach was the right sort of candidate to replace him one day.
Instead, it will be Guardiola versus Ten Hag, City against United this weekend on a day when any sort of friendship or respect will take a back seat.
For Guardiola, it is another opportunity to prove why he remains a true master; for Ten Hag, a chance to show that his apprenticeship has been put to good use.