Sources of law
Sources of law in its narrow sense means the origins of law, i.e. the binding rules governing human conduct. More generally, it means any premiss of a legal reasoning. Such sources may be international, national, regional or religious.
The term "sources of law" also refers to the sovereign or the state from which the law derives its force or validity. In civil law systems, one has only to look at the appropriate code; but in common law systems one needs to look at legislation (primary and secondary) and at the decided cases that comprise judicial precedent.
There are many factors of law that have contributed to the development of law and which are regarded as the sources of law. Legal customs, Divine right, Natural and legal rights, human rights, civil rights, and common law are often implied and unwritten sources of law that have been established over decades or centuries. Canon law and other forms of religious law form the basis for law derived from religious practices and doctrines or from sacred texts; this source of law is important where there is a state religion. Historical or judicial precedent and case law can modify or even create a source of law. The ultimate in written laws are the charter, the constitution, and the treaty, much of which form the foundation of modern legal systems. Legislation, rules, and regulations are often the source of laws which are codified and enforced by the legal system.
Governments may sign International Conventions and Treaties; but these normally become binding only when they are ratified. Most conventions come into force only when a stated number of signatories have ratified the final text. An international convention may be incorporated into a statute (e.g. Hague-Visby Rules in Carriage of Goods by Sea Act 1971; e.g. the Salvage Convention in the Merchant Shipping Act 1995). The Council of Europe’s European Convention on Human Rights is enforced by the ECHR in Strasbourg.
European Community Law
The European Union is special example of international law. European nations that join the EU thereby adopt all EC Law to date (the acquis communautaire), namely: treaty provisions, regulations, directives, decisions, and precedents. Member States become subject to “Brussels”  and to the binding precedent decisions of the Court of Justice of the European Union (or CJEU in Luxembourg. However, Brussels may only act and legislate in accordance with the EU treaties, and the CJEU's supremacy applies only in matters of EU law.
Legislation is the prime source of law. and consists in the declaration of legal rules by a competent authority. Legislation can have many purposes: to regulate, to authorize, to enable, to proscribe, to provide funds, to sanction, to grant, to declare or to restrict. A parliamentary legislature frames new laws, such as Acts of Parliament, and amends or repeals old laws. The legislature may delegate law-making powers to lower bodies. In the UK, such delegated legislation includes Statutory Instruments, Orders in Council, & Bye-laws. Delegated legislation may be open to challenge for irregularity of process; and the legislature usually has the right to withdraw delegated powers if it sees fit.
Most legislatures have their powers restricted by the nation's Constitution, and Montesquieu's theory of the separation of powers typically restricts a legislature's powers to legislation. Although the legislature has the power to legislate, it is the courts who have the power to interpret statutes, treaties and regulations. Similarly, although parliaments have the power to legislate, it is usually the executive who decides on the legislative programme. The procedure is usually that a bill is introduced to Parliament, and after the required number of readings, committee stages and amendments, the bill gains approval and becomes an Act.
Judicial precedent (aka: case law, or judge-made law) is based on the doctrine of stare decisis, and mostly associated with jurisdictions based on the English common law, but the concept has been adopted in part by Civil Law systems. Precedent is the accumulated principles of law derived from centuries of decisions. Judgements passed by judges in important cases are recorded and become significant source of law. When there is no legislature on a particular point which arises in changing conditions, the judges depend on their own sense of right and wrong and decide the disputes from first principles. Authoritative precedent decisions become a guide in subsequent cases of a similar nature. The dictionary of English law defines a judicial precedent as a judgement or decision of a court of law cited as an authority for deciding a similar state of fact in the same manner or on the same principle or by analogy. Another definition  declares precedent to be," a decision in a court of justice cited in support of a proposition for which it is desired to contend".
Compared to other sources of law, precedent has the advantage of flexibility and adaptability, and may enable a judge to apply "justice" rather than "the law".
Equity (England only)
Equity is a source of law peculiar to England and Wales. Equity is the case law developed by the (now defunct) Court of Chancery. Equity prevails over common law, but its application is discretionary. Equity's main achievements are: trusts, charities, probate, & equitable remedies. There are a number of equitable maxims, such as: “He who comes to equity must come with clean hands”.
Parliamentary Conventions (UK mainly)
(not to be confused with International Conventions)
Parliamentary Conventions are not strict rules of law, but their breach may lead to breach of law. They typically are found within the English legal system, and they help compensate for the UK's lack of a single written constitution. Typically, parliamentary conventions govern relationships, such as that between the House of Lords and the House of Commons; between the monarch and Parliament; and between Britain and its colonies. For instance, after the Finance Act 1909, the House of Lords lost its power to obstruct the passage of bills, and now may only delay them. The prerogative powers are subject to convention, and in 2010, the monarch's power to dissolve Parliament was abolished. Britain's tradition with its colonies is that they are self-governing (although, historically, not necessarily with universal suffrage).
Customs (England mainly)
A "General Custom" as a source of law is not normally written, but if a practice can be shown to have existed for a very long time, such as "since time immemorial' (1189 AD), it becomes a source of law.
A "ParticularCustom" (or "private custom") may arise and become a right with the force of law when a person, or a group of persons has from long usage obtained a recognised usage, such as an easement.
Books of Authority (England mainly)
Up until the 20th century, English judges felt able to examine certain "books of authority" for guidance, and both Coke and Blackstone were frequently cited. This old practice of citing only authors who are dead has gone; nowadays notable legal authors may be cited, even if they are still alive.
- Goltzberg, Stefan (2016). Les Sources du droit. Paris: Presses Universitaires de France. ISBN 2130748600.
- Slapper & Kelly - English Legal System - Routledge-Cavendish - 2008 - page 65
- "Dualist" jurisdictions require ratification of treaties; "monist" jurisdictions do not.
- For instance the Maritime Labour Convention entered into force on 20 August 2013, one year after registering 30 ratifications of countries representing over 33 per cent of the world gross tonnage of ships
- "Brussels" is the Commission, the Council of Ministers & the European Parliament acting in concert
- e.g.. such as Van Gend en Loos.
- Executive and judicial power is to be exercised by, respectively, the government and the courts.
- In the UK, the government sets out its programme in the Queen's Speech
- In the EU, only the Commission may initiate legislation
- In the UK, this requires the Royal Assent
- Mozley & Whiteley's Law Dictionary - E.R. Hardy Ivamy
- The Judicature Acts 1873-75 abolished the Courts of Chancery. Chancery Division of the High Court succeeds the old Courts of Chancery
- Edward Coke's Institutes of the Lawes of England, William Blackstone's Commentaries on the Laws of England, and similar texts.