In the early days of the Rome, citizens voted in the "comitia curiata." (Heads of families came together "cum-ire" in a gathering of thirty "curiae" or wards into which the three tribes had been divided.) It was the Curial Assembly (later the Centurial Assembly) that initially conferred upon the elected magistrates the "imperium" or right to employ the power and authority of the state to enforce the law governing the actions of individuals. (Note: The modern concept of "government" identifies component parts as the legislative, executive and judicial functions. Under "rule of law," the law and the Constitution define the limits of the power of the state to govern the actions of individuals.)
In its early days, Rome's only law-making authority was the "populus Romanus" assembled and voting in its "comitia." This body made "leges." Later, equal legislative authority was accorded to the "plebs Romana," assembled and voting in its "concilium." This body made "plebocita." These two kinds of enactments were equally legislative and were the only "laws of Rome."
The praetors were annually elected magistrates whose duties included the administration of the law courts. Magistrates were delegated the right or power to issue edicts. Edicts could be declared by the urban praetor (the praetor who gives justice between Roman citizens,) the "praetores peregrini," and provincial governors. Magistrates with equal imperium could quash one another's acts. The consuls, who had the highest imperium, were always entitled to exercise jurisdiction to quash that of the praetors.
Each praetor made it a practice to publish an "edicta praetorium" before entering office announced in the Forum and inscribed upon a wall. These stated the legal principles upon which the praetor proposed to act and judge during his year's term. When a law or clause was carried forward from one praetorian edict to the next, surviving for several years, it became a part of the "ius honorarium." By the time of Cicero, this ius honorarium or "law of offices" had displaced the Twelve Tables as the main text of legal instruction in Rome. However, a praetor could reverse decisions and contradict the principles of a predecessor. It was to that uncertainty that Hadrian in 117 AD instructed Julianus to unify all preceding ius honorarium in a Perpetual Edict, alterable only by the emperor.
The preator did not try anyone. He ascertained the exact nature of the dispute between litigants, issued a "formula" containing the precise questions that were to be tried, (ius dicere,) and appointed someone to try the action. His proceedings were "in iure," or "in presence of law officer," and might involve counsel for the parties and argument as to law and fact. When these proceedings were complete, the praetor's function was over. One or more persons were appointed to actually judge the action, itself.
Technically, the Romans recognized "custom" and historical tradition only as institutionalized in the "Twelve Tables" and the leges. The edicts supported, supplemented and corrected the civil law. [Papinian] Through their power of "imperium" or rule, praetors were allowed not only to interpret existing law, but to make new ones (Roman "Equity.")
During the period of Augustus, the provinces were divided into:
Provinces of Caesar that were ruled by "legati,"( or delegates of the Emperor,) who could did not have the power to delegate further, and
Those that were ruled by governors with their own individual imperium and powers of delegation, subject only to the overriding maius imperium of the Emperor.
The provincial governor was the sole independent jurisdictional authority in the provinces. He was the font of all procedural law, entitled to "cognoscere," or the actual trying of a case - including, summons, determination of issues, hearing of evidence, judgment and execution. Generally, he remanded civil suits between peregrines in the hands of local courts, although he was not required to do so unless restrained by a municipal charter. All suits involving a Roman citizen and all criminal cases came to the governor, although the Roman citizen retained the traditional right of appeal to the people - provocatio as populum, [which under the principiate became "appeal to Caesar.]
The emperor, (established with Augustus in 27 BC from the magistracy of the consul,) also had the right to issue edicts on legal affairs. But unlike that of the republican magistrates his power was life-long, so that his edicts were effective for a considerable time. Further, succeeding emperors usually observed the enactments of their predecessors. In the early principiate, the emperor asked for the concurrence of the Senate. The concurrence of the Senate eventually became a matter of course
The Roman senate served initially only in the capacity of an advisory council to the state's executive officers. Its role came to be exercised primarily in the field of foreign policy and dealings with the states and peoples outside of Rome. The resolutions or policy decisions of the senate were called "senatuconsulta."
The constitutions of the emperors began as linking together of the magistracies various executive and military powers. During the period of the "Principiate," the command of the elected emperor virtually replaced the vote of the people in their assemblies as the law-making authority. By the end of the period of the Principiate, it was stated: "That which had been decided by the emperor had the force of law." [Ulpian]
Senatuconsulta declaring policies of this period was most often proposed by the emperor in a formal speech and his speech was quoted without reference to the corresponding formal resolution made by the senate.
The "constitutiones principum" or statutes of the princes became a source of law in the second century. This took four forms:
The prince issued "edicta" by virtue of his "imperium" as an official of the city of Rome. These were valid throughout the Empire, but lapsed after the prince's death.
The prince declared "decreta" as a judge, and like other magistrates, this had the force of law.
Imperial "rescripta" were the prince's answers to inquiries. These were usually "epistulae" (letters) or "subscriptiones" (brief replies written under a question or petition.) Some rescripta, such as those of Trajan were incorporated into the laws of the Empire.
The "mandata" of the emperors were their directives to officials. In time these came to constitute a detailed code of administrative law.
After Augustus, legislation by the people was rare. The last known "law of the Roman people" was passed under the Emperor Nerva in AD 97. However, these were termed "leges rogatae," bills put to the people for ratification, rather than legislation initiating from the people.
Under certain circumstances, law could be created by the "responsa prudentium" where jurists sat at the Forum and gave legal opinions to all who asked (often lawyers and magistrates.) Their written replies, by unwritten custom, had force just short of law. Augustus gave such opinions legal force when the jurist had received from the Emperor the "ius respondendi" or right to give legal opinions; and the reply was sent under seal to the judge trying the case in point. By the time of Justinian, the responsa had become a vast body of opinion.
In 528 AD, the Eastern Roman (Byzantine) emperor, Justinian I, selected committees that collected, edited, and organized the scattered and sometimes contradictory legal materials from all these sources and published them as the Corpus Juris Civilis (Body of Civil Law). This is the form in which most Roman law has come down to us and was reintroduced in England by the Church during the Middle Ages.