Leges regiae, The Twelve Tables (Lex duodecim tabularum); other Leges publicae; Praetorian edicts; Senatus consulta; Imperial enactments collected in various codes, including the Theodosian Code which contains imperial enactments from AD 312-438; and literary sources such as the barbarian codes.
Gaius (2nd century AD); Ulpian (AD 170-223/4); Paulus (fl. c. AD 200), are particularly significant because the texts survived independently of the Justinian compilations, and therefore represent earlier ideas:
Gaius’ Institutes; Paulus’ Sententiae; Ulpian’s Regulae.
See entries such as Jurists, Institutes of Gaius, Paul, Ulpian in Oxford International Encyclopedia of Legal History
Justinian’s works are known collectively as the Corpus Juris Civilis (body of civil law) and consist of: the Digest (or Pandects), Institutes, Code and Novels.
The Digest, Institutes and Code are restatements or revisions of existing law. The Novels are Justinian’s subsequent enactments.
About 892 AD during the reign of Leo VI, Eastern Roman Emperor, a Greek abridgment of Justinian's laws was promulgated: Τà Βασιλικα