In The Spirit of the Laws Montesquieu defines liberty as "the tranquility of mind that results from the conviction that everybody has his security." This indicates that for Montesquieu, no less than the Founding Fathers, liberty could only arise if the appropriate institutions were in place, institutions that guaranteed the citizen's security under the rule of law. But this liberty that Montesquieu prizes so highly is not the right to do whatever we want, whenever we want; Montesquieu doesn't confuse liberty with license. Under the rule of law, liberty resides in "the power to do what one should will, and not to be compelled to do what one should not will to do." In other words, citizens must be allowed to do what they ought to do according to commonly-accepted laws and rules, but not be so compelled by any government institution.
Montesquieu further suggests that the best system of government for preserving liberty is one in which there is a separation of powers between the legislative, executive, and judicial branches. He famously misinterpreted the unwritten British constitution as providing for just such a system. However, it wasn't until the founding of the United States that Montesquieu's ideal system of government, and with it his ideal of liberty, was put into effect.