The Constitution of India explained

Article 14 guarantees equality before the law and equal protection of the laws. It permits reasonable classification but forbids arbitrariness — a principle the courts have extended far beyond its literal text to strike down capricious state action generally.

Article 15 prohibits discrimination on grounds of religion, race, caste, sex or place of birth, while permitting special provisions for women, children and socially and educationally backward classes — the constitutional basis for reservations. Article 16 applies the same logic to public employment.

Article 17 abolishes untouchability and makes its practice an offence — one of the few articles that directly criminalises conduct.

Article 19 guarantees six freedoms: speech and expression, assembly, association, movement, residence, and profession or trade. Each is subject to 'reasonable restrictions' on specified grounds, and the scope of what is reasonable is where most free speech litigation happens.

Article 21 has become the most consequential provision in the Constitution. Its text is spare — no person shall be deprived of life or personal liberty except according to procedure established by law — but since Maneka Gandhi (1978) the courts have read into it a right to live with dignity, and from that a right to privacy (Puttaswamy, 2017), to livelihood, to a clean environment, to health, to shelter, to legal aid and to a speedy trial. Article 21A added a right to free and compulsory education for children aged six to fourteen.

Articles 25 to 28 guarantee freedom of conscience and religion; Articles 29 and 30 protect the cultural and educational rights of minorities, including their right to establish and administer educational institutions.

Article 32 gives you the right to move the Supreme Court directly for enforcement of a fundamental right, and it is itself a fundamental right. Ambedkar called it the article without which the Constitution would be a nullity. Article 226 gives High Courts a wider power, covering fundamental rights and other legal rights.

The courts issue five writs: habeas corpus, to produce a detained person; mandamus, to compel a public authority to perform its duty; prohibition and certiorari, against lower courts and tribunals exceeding jurisdiction; and quo warranto, challenging a person's claim to public office.

Public interest litigation, developed from the late 1970s, relaxed the traditional rule that only an injured party may sue. Any public-spirited person can bring a matter affecting others unable to approach the court themselves. Courts have accepted letters and postcards as petitions.

PIL has produced substantial results — on bonded labour, prison conditions, environmental protection and food security — and substantial criticism, for drawing courts into policy questions and for occasional misuse.

In practice, most people's route to enforcement is a High Court writ petition under Article 226 rather than the Supreme Court, and free legal aid through the District Legal Services Authority is available to those who qualify, which includes all women and children, SC/ST members, and anyone below the prescribed income limit.

Part IV sets out Directive Principles of State Policy: goals for governance including a living wage, equal pay for equal work, free legal aid, village panchayats, protection of the environment, separation of judiciary from executive, and a uniform civil code.

The crucial difference is enforceability. Article 37 states that the Directive Principles are not enforceable by any court, but are nevertheless fundamental in the governance of the country and a duty of the state to apply. You cannot sue for a living wage under Part IV.

The relationship between Parts III and IV has been the central constitutional argument of independent India, most visibly in the litigation over land reform and property rights, which drove several amendments and ultimately produced the basic structure doctrine. The courts now read the two parts harmoniously, treating the Directive Principles as an aid to interpreting fundamental rights — which is largely how Article 21 came to include health, education and livelihood.

Part IVA adds Fundamental Duties in Article 51A, inserted by the 42nd Amendment in 1976: to abide by the Constitution, cherish the freedom struggle's ideals, protect sovereignty, defend the country, promote harmony, preserve heritage, protect the environment, develop scientific temper, safeguard public property, strive towards excellence, and provide education to one's children. They are not directly enforceable either, though courts have used them interpretively.

Article 368 sets out the amendment procedure. Most amendments require a majority of the total membership of each house and two-thirds of members present and voting. Amendments affecting federal features — the election of the President, the distribution of legislative powers, representation of states in Parliament, the judiciary — additionally require ratification by the legislatures of at least half the states.

The Constitution has been amended over a hundred times, far more often than most comparable constitutions, partly because it is so detailed that ordinary policy changes require amendment.

The decisive limit is judicial. In Kesavananda Bharati (1973), a thirteen-judge bench held by the narrowest margin that Parliament's amending power does not extend to altering the Constitution's basic structure. The Court has never issued an exhaustive list, but supremacy of the Constitution, the republican and democratic form of government, secularism, separation of powers, federalism, judicial review, free and fair elections and the rule of law have all been identified as basic features.

The doctrine was tested immediately: the 39th Amendment, which sought to place the Prime Minister's election beyond judicial challenge, was struck down in 1975, and parts of the 42nd Amendment passed during the Emergency were undone in Minerva Mills (1980).

Notable amendments include the 42nd (1976), which inserted 'socialist' and 'secular' into the Preamble and added the Fundamental Duties; the 44th (1978), which removed property from the list of fundamental rights and tightened emergency provisions; the 73rd and 74th (1992), which created local government; the 86th (2002), which made education a fundamental right; and the 101st (2016), which introduced GST.