A more thorough explanation:

Permutatio

Permutatio is a legal agreement between two parties to exchange goods or services. Once one party transfers ownership of their item to the other, the agreement becomes binding.

• John and Sarah agree to exchange their cars. John gives Sarah his car, and Sarah gives John her car. This is an example of permutatio.
• Tom and Jerry agree to exchange their houses. Tom gives Jerry his house, and Jerry gives Tom his house. This is also an example of permutatio.

These examples illustrate permutatio because they show two parties agreeing to exchange ownership of their items. Once the exchange is complete, the agreement becomes binding.