Following up, with more detailed history, from F.W. Maitland, Equity, also The Forms of Action at Common Law (1st edn, 2010), available on archive.org here:

link to archive.org

First to check that this edition of the book is in the (copyright) public domain under the “death plus seventy years” rule. The author and editors are the following:

Frederick William Maitland (1850–1906)

Alfred Henry Chaytor (1869–1931)

William Joseph Whittaker (1868–1931).

Thus this text, freely available on the Internet, should indeed be in the public domain. The quote is from pages 2–4.

[What follows below the tramlines is taken from Maitland’s Equity.]

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In Edward I’s day, at the end of the thirteenth century, three great courts have come into existence, the King’s Bench, the Common Bench or Court of Common Please and the Exchequer. Each of these has its own proper sphere, but as time goes on each of them attempts to extend its sphere and before the middle ages are over a plaintiff has often a choice between these three courts and each of them will deal with his case in the same way and by the same rules. The law which these courts administer is in part traditional law, in part statute law. Already in Edward I’s day the phrase ‘common law’ is current. It is a phrase that has been borrowed form the canonists—who used ‘jus commune‘ to denote the general law of the Catholic Church; it describes that part of the law that is unenacted, non-statutory, that is common to the whole land and to all Englishmen. It is contrasted with statute, with local custom, with royal prerogative. It is not yet contrasted with equity, for as yet there is no body of rules which bears this name.

One of the three courts, namely the Exchequer, is more than a court of law. From our modern point of view it is not only a court of law but a ‘government office,’ an administrative or executive bureau; our modern Treasury is an offshoot from the old Exchequer. What we should call the ‘civil service’ of the country is transacted by two great offices or ‘departments’: there is the Exchequer which is the fiscal department, there is the Chancery which is the secretarial department, while above these rises the king’s permanent Council. At the head of the Chancery stands the Chancellor, usually a bishop, he is we may say the king’s secretary of state for all departments, he keeps the king’s great seal and all the already great mass of writing that has to be done in the king’s name has to be done under his supervision.

He is not yet a judge, but already he by himself or his subordinates has a great deal of work to do which brings him into a close connexion with the administration of justice. One of the duties of that great staff of clerks over which he presides is to draw up and issue those writs whereby actions are begun in the courts of law—such writs are sealed with the king’s seal. A man who wishes to begin an action must go to the Chancery and obtain a writ. Many writs there are which have been formulated long ago; such writs are writs of course (brevia de cursu), one obtains them by asking for them of the clerks—called Cursitors—and paying the proper fees. But the Chancery has a certain limited power of inventing new writs to meet new cases as they arise. That power is consecrated by a famous clause of the Second Statute of Westminster authorising writs in consimili casu. Thus the Chancellor may often have to consider whether the case is one in which some new and some specially worded writ should be framed. This however is not judicial business. The Chancellor does not hear both sides fo the story, he only hears the plaintiff’s application, and if he grants a writ the courts of law may afterwards quash that writ as being contrary to the law of the land.

But by another route the Chancellor is brought into still closer contact with the administration of justice. Though these great courts of law have been established there is still a reserve of justice in the king. Those who can not get relief elsewhere present their petitions to the king and his council praying for some remedy. Already by the end of the thirteenth century the number of such petitions presented in every year is very large, and the work of reading them and considering them is very laborious. In practice a great share of this labour falls on the Chancellor. He is the king’s prime minister, he is a member of the council, and the specially learned member of the council. It is in dealing with these petitions that the Chancellor begins to develop his judicial powers.

In course of time his judicial powers are classified as being of two kinds. it begins to be said that the Court of Chancery, ‘Curia Cancellariae’—for the phrase is used in the fourteenth century—has two sides, a common law side and an equity side, or a Latin side and an English side. Let us look for a moment at the origin of these two kinds of powers, and first at that which concerns us least.

(1) Many of these petitions of which I have spoken seek justice not merely from the king but against the king. If anybody is to be called the wrong doer, it is the king himself. For example, he is in possession of land which has been seized by his officers as an escheat while really the late tenant has left an heir. Now the king can not be sued by action—no writ will go against him; the heir if he wants justice must petition for it humbly. Such matters as these are referred to the Chancellor. Proceedings are taken before him; the heir, it may be, proves his case and gets his land. The number of such cases, cases in which the king is concerned, is very large—kings are always seizing land on very slight pretexts and forcing other people to prove their titles. Gradually a quite regular and ordinary procedure is established for such cases—a procedure very like that of the three courts of Law. The proceedings are enrolled in Latin—just as proceedings of the three courts of law are enrolled in Latin (hence the name ‘Latin side’ of the Court of Chancery)—and if a question of fact be raised, it is tried by jury. The Chancellor himself does not summon the jury or preside at the trial, he sends the question for trial to the King’s Bench. All this is by no means unimportant, but it does not concern us very much at the present time.

(2) Very often the petitioner requires some relief at the expense of some other person. He complains that for some reason or another he can not get a remedy in the ordinary course of justice and yet he is entitled to a remedy. He is poor, he is old, he is sick, his adversary is rich and powerful, will bribe, or will intimidate jurors, or has by some trick or some accident acquired an advantage of which the ordinary courts with their formal procedure will not deprive him. The petition is often couched in piteous terms, the king is asked to find a remedy for the love of God and in the way of charity. Such petitions are referred by the king to the Chancellor. Gradually in the course of the fourteenth century petitioners, instead of going to the king, will go straight to the Chancellor. will address their complaints to him and adjure him to do what is right for the love of God and in the way of charity. Now one thing that the Chancellor may do in such a case is to invent a new writ and so provide the complainant with a means of bringing an action in a court of law. But in the fourteenth century the courts of law have become very conservative and are given to quashing writs which differ in material points from those already in use. But another thing that the Chancellor can do is to send for the complainant’s adversary and examine him concerning the charge that has been made against him. Gradually a procedure is established. The Chancellor having considered the petition, or ‘bill’ as it is called, orders the adversary to come before him and answer the complaint. The writ whereby he does this is called a subpoena—because it orders the man to appear upon pain of forfeiting a sum of money, e.g., subpoena centum librarum. It is very different from the old writs whereby actions are begun in the courts of law. They tell the defendant what is the cause of action against him—he is to answer why he assaulted and beat the plaintiff, why he trespassed on the plaintiff’s land, why he detains a chattel which belongs to the plaintiff. The subpoena, on the other hand, will tell him merely that he has got to come before the Chancellor and answer complaints made against him by A. B. Then when he comes before the Chancellor he will have to answer on oath, and sentence by sentence, the bill of the plaintiff. This procedure is rather like that of the ecclesiastical courts and the canon law than like that of our old English courts of law. It was in fact borrowed from the ecclesiastical courts, not from their ordinary procedure but from the summary procedure of those courts introduced for the suppression of heresy. The defendant will be examined upon oath and the Chancellor will decide questions of fact as well as questions of law.

[Maitland is embarked on the story of the origins of Equity, as a distinct body of jurisprudence with procedures and remedies distinct from those of the common law. The story continues, but I will break off here.]