People have this "folk" definition of monopoly saying that a monopoly is 100% total market domination. Maybe because this is the win condition for the board game Monopoly? I think this idea is a serious impediment to addressing anti-competitive behavior in America.

When antitrust proceedings starts against Standard Oil in 1904, they had 91% market share [0]. By 1911 when they were broken up they were down to 64%, but that’s because the case progressed. And after the breakup they had 0%.

But I think if Standard had 64% in the beginning and there was a competitive market, they would not have been broken up.

[0] https://www.visualcapitalist.com/chart-evolution-standard-oi...

Which is why I said down to, they didn’t get to say we’re below the magic X% number can you please leave us alone. But presumably if their market share had fallen to 10% that would have happened.

As to when the case started, it’s impossible to know what the minimum threshold was. It could just as easily been well below 64% as people where in monopoly busting push at the time.