How did the enlightenment philosophers use the idea of a contract in their political thinking?

Enlightenment philosophers used the idea of a contract as
a way to imagine how government got its start in human society and to imagine the basis
of government's legitimacy.

To Enlightenment thinkers like
Locke and Rousseau, human beings entered into a form of contract with one another when
they created government.  They promised to obey their government.  In return, the
government promised to protect the people's basic human rights.  Government was, to
these thinkers, legitimate because of this contract.  If the government broke its part
of the contract, if it did not protect the rights of the people, the contract was void
and the people had the right to rebel.

In this way, these
thinkers used the idea of a contract to understand the origins of government and the
source of its legitimacy.