During the Spring and Autumn Period and the Warring States Period in China, the princes exchanged hostages in order to win the trust of each other, which was called proton. This is usually mortgaged to other countries by the son of a vassal as a proton. In addition to the equal exchange of hostages, there are also cases where weak countries unilaterally emit protons when they want to be strong.
Protons usually live in enemy countries or the capitals of other countries. Although they have their own houses, their activities and personal freedom are restricted. Proton is an important diplomatic strategy of China in ancient times, which is often seen in the submission of small countries to big countries. Since then, diplomacy has been subject to enemy countries everywhere.