In 528, the Roman emperor Justinian I undertook a great project - the codification and standardification of more than a millennium of Roman law. The resulting work was called the corpus iuris civilis, or "the body of civil law."
The corpus consists of three volumes, with later additions of new laws called Novels. Latin had long been the language of law in both halves of the empire, and thus the three volumes of the corpus were all written in that language. However, in Constantinople Latin gradually became associated with the "barbarians" who had taken over control of the western empire, and fell out of favor; by the end of Justinian's reign it had been largely abandoned, and thus most of the Novels were written in Greek.
The Codex Justianus was the first volume of the corpus, completed in 529 by the quaestor Trebonian. It was primarily a compilation of imperial law (ius principale, literally "principal law,") garnered from the Codex Theodosianus and private collections such as the Codex Gregorianus and the Codex Hermogenianus. Essentially, the Codex was made up of the constitutiones (decrees) of all emperors since Hadrian, organized into 12 books. The subsequent reordering of the laws in the second volume of the corpus rendered the first edition - known as the codex vetus, or "old book" - obsolete, however. The codex vetus was replaced by an updated volume in 534, and has since disappeared from the historical record.
The Digest (or the Pandects in Greek) was the second and more ambitious work of the corpus. It was completed in 533, again under the aegis of Trebonian, and consisted of the writings of 39 of the most famous Roman jurists (the ius vetus, or "old law,") in 50 books. However, it was not just a rote compilation - the writings were arranged by subject, and amended, clarified, or revised as necessary. Justinian's instructions to Trebonian were recorded in the Digest's prologue:
We desire you to be careful with regard to the following: if you find in the old books anything that is not suitably arranged, superfluous, or incomplete, you must remove all superfluities, supply what is lacking, and present the entire work in regular form, and with as excellent an appearance as possible. You must also observe the following, namely: if you find anything which the ancients have inserted in their old laws or constitutions that is incorrectly worded, you must correct this, and place it in its proper order, so that it may appear to be true, expressed in the best language, and written in this way in the first place; so that by comparing it with the original text, no on can venture to call in question as defective what you have selected and arranged.
The Digest was thus nothing less than the reorganization of a thousand years of learned but often contradictory legal opinion into an organized body of law.
The Institutes (Institutiones in Latin) was the third volume of the corpus, and consisted of the basics of Roman law without the encumbrance of supplementary matter or commentary. Justinian appointed a commission consisting of Trebonian, Theophilus, and Dorotheus to produce the work as a textbook, since either the Codex nor the Digest were particularly suitable for reference or instructional purposes. The authors based the Institutes upon the Commentarii and the Res Quotidianae of Gaius, which appear to have been produced during the rule of Antoninus Pius or his successor, Marcus Aurelius. Both the works of Gaius and the Institutiones deal primarily with the ius privatum, or "personal law" - the judicia publica, or "public court" was left to the Codex and Digest. The Institutiones were completed in the same year (533 CE) as the Digest, and released concurrently; Theophilus also produced a Greek paraphrase of the work, which has also survived to the present day.
So why should you care?
There are only three widespread systems of law in the world today - the Anglo-Saxon "common law," used in the U.S. and Britain, the Islamic system of Sharia - and Roman law, which forms the basis of the legal systems of most of Europe, Scotland, Quebec, and Louisiana.
As Europe began to emerge from the so-called "Dark Ages," the inheritors of the empire began to look beyond early Germanic and Viking law - which rested mainly on concepts such as trial by ordeal and the weregeld. As the kings and princes of these new kingdoms began to grapple with the task of creating complex laws for increasingly complex societies, they discovered that a ready-made system was already in their possession - the corpus iuris civilis. At various times and in various places, the Institutes were given legal force, and the volume became both textbook and statute; in fact, the Roman-Dutch Law - the basis of the South African legal system - is a branch of the revived Roman legal system that served Europe well into the 18th and 19th centuries.
Furthermore, while the standard Roman law of the corpus has been replaced by modern law codes, these codes consist largely of the old Roman rules placed into a more modern framework and augmented with new material. The corpus iuris civilis, then, is the seminal work of a legal system that has endured for more than two millennia and shapes the destiny of the western world even to this day. Justinian (and Trebonian) would be proud.
Sources:
Collins, Roger, "Early Medieval Europe 300-1000." St. Martin's Press, New York, 1991.
"Corpus Iuris Civilis," from The Roman Law Library, http://www.upmf-grenoble.fr/Haiti/Cours/Ak/.
Halsall, Paul, "The Institutes of Justinian, 535 CE," from the Internet Medieval Sourcebook, http://www.fordham.edu/halsall/.
"Illuminating the Law: Legal Manuscripts at the Fitzwilliam Museum, Cambridge," http://www.fitzmuseum.cam.ac.uk/msspb/exhibit/law/texts.htm.
Justinian I, "Digest." Translation by Monro, C.H., The Digest of Justinian Cambridge University Press, Cambridge, 1904, via the Medieval Sourcebook, http://www.fordham.edu/halsall.
Long, George, "Institutiones," from A Dictionary of Greek and Roman Antiquities. John Murray, London, 1875.
Peck, Harry, "Corpus Iuris Civilis," from Harpers Dictionary of Classical Antiquities. New York, Harper and Brothers, 1898.
Pennington, Ken, "Justinian's Corpus Iuris Civilis," http://www.fitzmuseum.cam.ac.uk/msspb/exhibit/law/texts.htm.
Rufner, Thomas, "Roman Law: Questions and Answers," http://www.jura.uni-sb.de/Rechtsgeschichte/Ius.Romanum/RoemRFAQ-e.html. See the Roman law node for the complete text of this article.