Pat owns and operates a gym whose main competitor is Jess, who runs another gym three streets away. They cater to the same clientele. Pat and Jess keep trying to undercut each other in order to attract customers. One evening Pat sees Jess in the local coffee shop and sits down to talk. They discuss how difficult it is to do upgrades and make money with the prices they are charging. Pat winked at Jess and said, "You know, charging $$80$ per month would enable a good profit margin." The next day Pat started charging $$80$ per month, as did Jess. Customers started complaining particularly after another coffee drinker and gym customer who had been sitting at a table near Pat and Jess made it known that they had seen the two talking there. Pat and Jess deny any agreement to set the same price for gym membership. They avow that the decision was made by each of them independently.

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Refer to Fact Pattern $16-1.$ Ferris, a first$-$year law student and gym member, is suspicious but isn$$t sure what violation to claim. Which of the following is the correct term for an illegal agreement by providers of the same good or service to charge a set fee?

Vertical price fixing

Substantial price fixing

Predatory price fixing

Horizontal price fixing