Section 1 of Article I vests all legislative powers of the federal government in a bicameral Congress, consisting of a Senate and a House of Representatives. This structure was a crucial compromise during the Constitutional Convention of 1787, balancing the demands of populous states (who favored proportional representation) and smaller states (who demanded equal representation). By splitting the legislative authority, the Framers sought to establish an internal check within the lawmaking branch itself, ensuring that no single faction could easily dominate the legislative process.

Section 2 establishes the House of Representatives, often termed the 'People's House.' Members are elected every two years directly by the citizens, ensuring high responsiveness to popular sentiment. Qualifications require a Representative to be at least 25 years old, a U.S. citizen for seven years, and an inhabitant of the state they represent. Representation is apportioned based on population, determined by a decennial census. This section also grants the House the sole power of impeachment, acting as a grand jury for federal officials accused of high crimes and misdemeanors.

Section 3 governs the Senate, designed to be a more deliberative, stable body representing state interests. Originally chosen by state legislatures (amended by the 17th Amendment to popular vote), Senators serve staggered six-year terms, with one-third of the body up for election every two years. Each state receives exactly two Senators. A Senator must be at least 30 years old, a citizen for nine years, and a resident of their state. The Vice President serves as President of the Senate but only votes to break a tie. Crucially, the Senate holds the sole power to try all impeachments, requiring a two-thirds majority to convict.

Sections 4, 5, and 6 control congressional operations, elections, and member privileges. States retain primary authority over the 'Times, Places and Manner' of elections, though Congress may alter these regulations. Each house judges its own elections and qualifications, requires a majority quorum to conduct business, and must keep an official journal. Members are protected from arrest during legislative sessions (except for treason, felony, or breach of the peace) and enjoy absolute immunity for speech or debate within Congress, protecting them from executive overreach.

Section 7 details the precise mechanism of lawmaking and the presidential veto, a cornerstone of the checks-and-balances system. All bills for raising revenue must originate in the House of Representatives. Once a bill passes both chambers in identical form, it is presented to the President. If approved, it becomes law. If vetoed, it returns to the originating house with objections. Congress can override a presidential veto with a two-thirds majority in both chambers. If the President takes no action for ten days (excluding Sundays), the bill becomes law automatically, unless Congress has adjourned, which results in a 'pocket veto.'

Section 8 enumerates the specific powers granted to Congress, outlining the scope of federal authority. These include the power to lay and collect taxes, borrow money, regulate commerce with foreign nations and among states (the Commerce Clause), coin money, establish post offices, and constitute federal courts. It grants Congress the exclusive power to declare war, raise and support armies, and provide a navy. Section 8 concludes with the 'Necessary and Proper Clause' (or Elastic Clause), authorizing Congress to make all laws required to execute its enumerated powers, providing the Constitution with vital adaptability.

Sections 9 and 10 impose limits on federal and state power, respectively. Section 9 protects individual liberties by prohibiting the suspension of the Writ of Habeas Corpus except during rebellion or invasion, banning Bills of Attainder (legislative punishments without trial), and outlawing Ex Post Facto laws (retroactive criminal laws). It also bans export taxes on states and prohibits titles of nobility. Section 10 strictly restricts states from conducting foreign diplomacy, coining money, entering treaties, emitting bills of credit, or passing laws that impair contractual obligations, cementing national unity.