At what point does a balance of power truly swing? What is the real gauge of a pendulum moving from one side to the other, from black to white, or, in the case of football in Manchester: red to blue?
As United and City prepare to lock horns in the 175th Manchester derby on Sunday, it is a question that applies, both in the here and now, but also longer term.
City, eight points clear at the top of the Premier League, will nail the pendulum firmly to the blue side in terms of the immediate battle for the title if they win at Old Trafford and move 11 points clear of Jose Mourinho's United.
But City's recent record in Premier League derbies against United -- they have lost just two of nine since Robin van Persie's last minute free kick sealed a 3-2 United win at the Etihad in December 2012 -- suggests they also hold the upper hand when it comes to local pride.
Historically, and this is where United win every element in the Mancunian Tale of the Tape, United are the top dogs, with 72 victories in all competitions compared to City's 50 wins. The same applies to trophies, attendances, appearances in finals -- you name it, the long-term balance of power is held by United.
But it is now approaching 10 years since Sheikh Mansour bin Zayed al Nahyan arrived at City, in September 2008, to transform the club from perennial also-rans into one with aspirations of winning the Champions League. That moment was when a line in the sand was drawn.
City took time to build, but they were champions less than four years after the Abu Dhabi money began being pumped into every aspect of the club.
So if we take this decade to assess who holds the balance of power in Manchester now, Sunday's game becomes a key milestone. United certainly need to strike a blow against City, both in terms of the title race and their own derby pedigree, by winning at the weekend.
Mourinho's men cannot afford to fall 11 points behind, but a draw is also of little value with Pep Guardiola's team already eight points ahead of the pack and unbeaten domestically.
Slowly but surely, City have begun to eclipse United in the Premier League and it is not just on derby day that they have started to assert themselves. Since Sir Alex Ferguson retired as United manager at the end of the 2012-13 season, with the club having just secured a 20th league title, City have not finished below their neighbours in the Premier League table.
If City win this season's title, it will be five years in succession -- and six times out of seven -- that they have ended a campaign as top dogs in the city. A sustained period of dominance.
Yet despite all of the progress at the Etihad Stadium, they have been outscored by United when it comes to trophies since 2010. And this in a decade that has seen United battle to overcome the trauma of Ferguson's departure and the turmoil which followed, particularly under David Moyes and Louis van Gaal.
United have endured a turbulent time since 2010, with the divisive green-and-gold campaign against the club's owners, the Glazer family, followed by two attempts by Wayne Rooney to leave at the peak of his powers; Ferguson's retirement, the Moyes disaster, Van Gaal's dismal football and the difficulties of Mourinho's first season last year.
Yet throughout all of the above, United have won six major honours during this decade (not counting Community Shields) and City have won five.
If a club's status is measured by its success, then United remain ahead of City, both historically and in the short-term of this decade. But if they are to keep their noses ahead of City in the silverware stakes, they simply have to win on Sunday. Otherwise, City will be on course for the title and another year finishing ahead of United.
Avoiding defeat on Sunday will see City extend their impressive run of form against United and they could even end their rival's 40-game unbeaten home run, which dates back to their derby defeat against City back in September 2016.
If United win, they will chip away at City's lead, perhaps even eat into the confidence of Guardiola's players, and retain hopes of overhauling the Blues to win the title for the first time in the post-Ferguson era.
Will Sunday signal a resurgent United gearing up for a charge to the top? Or will it see City consolidate their current position as Manchester's No. 1 team?
It is well-poised and there is much more at stake than just three points.