Capitalization style for words and phrases related to legislation and international agreements is fairly straightforward, but here are some notes about treatment:
The phrase “US Constitution” (or “United States Constitution,” though the form with initials alone is sufficient) should be capitalized as such, as should names of state constitutions (“the California Constitution,” for example), but the word on its own is lowercased even as a subsequent reference to a specific document. The same is true of a word for components of a constitution, such as article.
Names of specific amendments to the US Constitution are capitalized, but whether words or numerals are used to indicate them is contingent on which authority is used: The Chicago Manual of Style, the style manual of record for book publishers, calls for generally spelling out numbers up to one hundred, but the Associated Press Stylebook, which prescribes style for newspapers (some magazines and many Web sites adhere to it as well), uses numerals for 10 and up. So, write “Thirteenth Amendment” or “13th Amendment” according to the style your self-selected or externally appointed style guide recommends.
Proposed amendments to the Constitution are often identified by their chief proponent (for example, “the Bricker Amendment”) or their aims (“the Equal Rights Amendment,” though some people argue that because there is no such amendment, only a movement to pass one, it should not be validated with capitalization).
Bills and Acts
A proposal for a new law enacted by the US Congress is offered as a bill. A bill proposed in the House of Representatives is given the body’s initials and a number (HR 99), followed by the name of the bill; a Senate is identified similarly (S 13). (As with US and other abbreviations, the initials are often followed by periods, but this style is unnecessary.) This style isn’t exactly mirrored in state and local legislation; for example, in the California Assembly, the local equivalent of the House of Representatives, a bill is abbreviated AB (for “Assembly Bill.”)
If passed, the bill becomes an act, such as the Don’t Ask, Don’t Tell Repeal Act of 2010. (Note that the year of enactment is often but not always part of the official name of the act.) In generic usage, even to a specific act, the word act is lowercased, though many legislative bodies and associated publications capitalize it when it refers to a specific act, as in “The Act would reverse a long-standing military policy that discriminates against gay service personnel.”
Many other names for legislation exist, including code, ordinance, and statute. These words are capitalized as part of the name of a body of laws, such as “Civil Code” or “Municipal Code,” but are otherwise lowercased.
Treaties and Such
Nomenclature for treaties includes formal and informal styles. For example, one notable example’s formal name is the Anti-Ballistic Missile Treaty, but the treaty associated with the Strategic Arms Limitation Talks is informally called the SALT treaty (with treaty lowercased because that is not the official name).
A similar international understanding is referred to as an agreement, as in “the Sykes-Picot Agreement.” Then there is an armistice, which is merely a cessation of hostilities that may or may not be followed by a peace treaty. Many notable armistices have occurred, requiring specific nomenclature such as “the Korean War Armistice Agreement,” but the default event, that formalizing the end of World War I, is referred to simply as “the Armistice.”