Saturday's Manchester derby is the first Premier League meeting between Jose Mourinho and Pep Guardiola but they've faced each another plenty of times in the past.
What, however, have we learned from those matches and will anything gleaned be relevant when United host City?
Both managers make specific plans for this opposition
Mourinho's contests against possession-based managers are often simple, because the approach generally does not change from game to game. This is partly why he has been so successful over the years against Arsene Wenger; Mourinho devised a plan and never needed to readjust.
Guardiola, though, is a different challenge. Although he has a defined philosophy and wants to play on the front foot, he scouts opponents extensively and makes specific plans to exploit their weaknesses.
Arguably Guardiola's greatest tactical triumph came in Barcelona's 3-1 victory at Real Madrid in December 2011, when he tricked Mourinho into thinking he was using a four-man defence, before shifting to more of a hybrid system that featured Dani Alves pushing forward down the right, Carles Puyol moving to full-back and Sergio Busquets playing centre-back and central midfield simultaneously. It was genius.
Mourinho's moves are more defensive. When facing Guardiola's Barca as Inter manager in 2009-10, he played defender Cristian Chivu on the left of midfield to shut down Alves. To combat the Brazilian while at Real, Mourinho generally played Cristiano Ronaldo upfront or on the opposite flank, with a more disciplined player tracking the Barca right-back. Mourinho would also use Lassana Diarra to constrict Lionel Messi's space.
In general, Guardiola changes things to get his key players into space, while Mourinho changes things to block opponents. That might define the tactical battle on Saturday.
Mourinho has struggled to include a playmaker
During the frequent meetings between Guardiola's Barcelona and Mourinho's Real Madrid, the latter discovered he was generally unable to use chief playmaker Mesut Ozil in his favoured No. 10 position.
The first time the two managers met in El Clasico was in November 2010. Mourinho asked Ozil to mark Xavi Hernandez, a reasonable request given Ozil was accustomed to a left-of-centre attacking midfield role and Xavi played in a deep right-of-centre midfield role.
But Xavi got the better of the duel by moving into a higher position, where Ozil would have been uncomfortable, and dictated the play from an advanced role. It's partly why Xavi found himself uncharacteristically opening the scoring from inside the box. Barcelona won 5-0.
Ozil was sacrificed at half-time in that game and thereafter rarely played as a No. 10 in El Clasico as Mourinho preferred midfielders, who could do a defensive job. Sometimes Ozil was left out, other times he was accommodated on the right because Barcelona didn't have a particularly good attacking left-back.
Who might Mourinho disappoint this weekend? Juan Mata is the most Ozil-esque player and is accustomed to setbacks under Mourinho, while Henrikh Mkhitaryan hasn't yet commanded a regular place. But could Wayne Rooney, afforded the freedom to play where he wanted by Sam Allardyce for England because he seems unable to play a set role, also be sacrificed?
Ibrahimovic was key in their most famous tie
When Mourinho's Inter defeated Guardiola's Barcelona over two legs in the 2009-10 Champions League semifinal, the defining image was Mourinho whispering in Guardiola's ear as the Camp Nou boss gave instructions to Zlatan Ibrahimovic. What was said? We don't know, but it's unlikely to have been helpful.
Ibrahimovic was the decisive factor in the tie, with Guardiola making a rare tactical error in starting the striker in both legs, having rushed him back when not fully fit. It meant Barcelona's passing play changed: They switched to a surprisingly direct game, which played into the hands of Inter's immobile centre-backs Walter Samuel and Lucio.
Guardiola substituted Ibrahimovic in both legs and switched each time to his classic 4-3-3 formation with a trio of quick, dynamic attackers. However, Inter simply retreated to deny space and Barcelona ended up shoving Gerard Pique upfront.
Ibrahimovic should have been the backup plan then but will be Man United's Plan A against City. Perhaps Mourinho will be whispering in his ear, reminding how Guardiola struggled to accommodate him. He could be crucial on derby day.
Guardiola turned to quicker forwards
Often, Guardiola's centre-forward is a decoy as much as a striker. Although he experimented with Ibrahimovic, his default setting was using Messi as a false nine. However, towards the end of his Barcelona reign he generally preferred to use Alexis Sanchez as his most advanced player against Mourinho and Madrid.
Rather than the Chilean dropping deep into his favoured No. 10 role, Sanchez played on the shoulder of the last defender, stretching play and creating space between the lines. This is particularly interesting, given Guardiola is without the suspended Sergio Aguero this weekend.
Kelechi Iheanacho is the obvious replacement and certainly offers speed, but Raheem Sterling could well be deployed upfront to make runs into the channels and allow others to thrive in the space behind.
Mourinho looks to control the centre
Teams managed by Mourinho generally defend narrowly, particularly against possession-based opponents. He's highly wary of allowing dangerous playmakers time to thread passes between defenders and therefore his teams are compact from flank to flank, as well as from front to back.
That made sense against Guardiola's Barcelona because he needed to shut down Busquets, Xavi and Andres Iniesta. But what will Mourinho do against Guardiola's Man City?
Yes, the threat of David Silva and Kevin De Bruyne in central midfield roles can't be ignored, but Guardiola has responded to opponents playing narrow and often asks his teams to pass through the channels instead.
When full-backs come inside to become midfielders, it allows the centre-backs space in the channels and the wingers stretch the play, rather than coming inside like Guardiola's Barca wide forwards.
There's a sense that Mourinho, for all his tactical nous, has been left behind the times with his defensive plans. Guardiola continues to evolve -- can Mourinho respond?