After billionaire Elon Musk said he was terminating his acquisition of Twitter, the social media company countered by citing a contract provision that’s often called upon when one party tries to back out of a deal.

The clause, known as specific performance, is often used in real estate cases to prevent buyers and sellers from calling off deals without good reason. But it’s also included in corporate merger agreements as a way to force a buyer or seller to close on a deal, barring material breaches such as fraud.