Locke starts off by recognizing that, in any civil society, situations will arise that have to be dealt with before the legislative can be assembled to provide laws for them. In these instances, the executive may exercise executive prerogative, or simply "good judgment." The executive is qualified to take actions that are outside the framework of the laws (not breaking them, just not provided for by them), if their actions advance the society's best interest. He defines this prerogative as "nothing but the power of doing public good without rule."
In the paternal societies discussed earlier, law was de facto, and rule was based on executive prerogative. Locke quickly corrects a possible misunderstanding that could arise from this description: even though all laws stemmed from executive prerogative, we cannot then say that the people, or the legislative, encroaches executive prerogative by passing laws to which the executive must be beholden. Encroachments can only be made on the public good, not on executive privilege or rights--the executive only has power inasmuch as the people invest in it. Prerogative, rather, is a trust placed by the people in the executive, which the executive is free to use so long as it uses it fairly.
A good leader will be tacitly allowed a large amount of prerogative by his people if his judgments tend to benefit everyone. Thus, Locke notes that "the reigns of good princes have been always most dangerous to the liberties of their people." The danger lies in the threat of a successor who, upon seeing the freedom his predecessor was allowed, will claim the same freedoms and rights based on precedent, and abuse power. In these cases, it can be difficult for the people to wrest the power back from the new offending leader, for he has taken as a right what is in fact a trust.
So, who judges when a leader has overstepped his prerogative? When the people come into conflict with some part of their government, no judge presides. Instead, the people can and must invoke "that ultimate determination to themselves which belongs to all mankind . . . whether they have just cause to make their appeal to heaven" and act against the executive in these cases.
Chapter 15 is a condensation of Locke's previous discussion of the differences between paternal, political, and despotical power. Paternal power is power that parents have over their children until they reach the age of reason (this power does not cover their property). Political power is the power that each individual in a society consents to submit to the commonwealth for the protection of their property. And despotical power is absolute, arbitrary power of one person to take the life and property of another against their will. Thus, nature gives parents paternal power, consent yields political power to the commonwealth, and forfeiture (unwillingly) gives a tyrant despotical power over his subjects.
The initial compromise of legislative power--that the executive may act without the explicit legal consent of the legislative--is deftly justified by Locke's explanation that the executive acts on behalf of the common good. Thus executive prerogative upholds the most fundamental tenet of the state--its preservation of the state. Furthermore, executive prerogative demonstrates the trust that must exist between the people and the executive (we saw mention of this in the executive's right to convene, adjourn, and alter the legislative, in Chapter 13). Locke makes it very clear, by contrasting the civil society with the paternal state, that the executive prerogative is not a right, but a duty of the executive, and that the people always retain the power to replace the executive.