The majority, upon entering into a commonwealth, get to choose their form of government. They may choose a democracy, in which case they retain the legislative powers for themselves, an oligarchy, in which they submit that legislative power to a few select persons, or a monarchy, in which they give power to a single person. The monarchy may be hereditary, if it passes from the ruler to his son, or elected, if a new ruler is elected by the majority's decision whenever the old ruler dies. The majority always has the power to change types of government. The placement of legislative power defines the type of government since legislative power is the supreme power within a civil state.
Locke then notes that by "commonwealth" he does not particularly mean democracy; rather he uses the term to underscore the point that the community, regardless of its form of government, exists for the commonwealth, for the good of all.
Chapter 11 is devoted to a study of the legislative power, which Locke has identified as the most important part of the government. The first rule of the legislative power is the preservation of the society. No one may challenge the power of the legislative body, or pass laws of their own; all such power is invested in this body by the majority (the majority can, of course, challenge the legislative in some instances). Every member of society must adhere to the laws laid down by the legislative body. The limits to the power of the legislature include the following: the legislation must govern by fixed "promulgated established laws" that apply equally to everyone; these laws must be designed solely for the good of the people; and the legislative must not raise taxes on the property of the people without the people's consent.
Here, Locke brings up what will be a constant concern: long-term office holders. This rule becomes particularly important when the legislature's members hold their positions for long periods of time, or even life; in these instances, they may think of themselves as a body separate from society, and start working for their own best interests rather than for society's. The legislation does not have the power to transfer its power--it cannot give the right to make laws to anyone else-since the people's majority have placed this power with the legislative, and the majority's will, being the only force more powerful than the legislature, cannot be contradicted.
Locke has laid down the groundwork for a civil society, and now he describes the best structure for that civil society. He starts with the legislature because, in Locke's model, the legislature is the supreme power in the government: it issues the laws that the executive must enforce, and that the judicial branch must use as a measure of justice.
Although today we may associate the ideals Locke espouses with democratic government, he was not solely focused on democracy, by any means. In his list of types of government, he does not favor any one above the others (although we know of his cautions about absolute monarchies)--as long as they adhere to his rules, they remain equally valid in his view.