Gajus Gails, or Cains, a Roman jurist, who flourished in the 2d century of our era, during the reigns of Hadrian, Antoninus Pius, and Marcus Aurelius. Of his personal history little or nothing is known, and even the spelling of his name has been the subject of controversy. From the references contained in the Digest it appears that he was the author of more than 15 works, of which the Institutes was by far the most important. This is supposed to have been the first work of the kind not compiled from previous sources, and to have afforded the first instance of a popular manual of Roman law in the sense of modern elementary text books. After a lapse of four centuries from its publication it was incorporated almost bodily into the celebrated Institutes prepared by the order of Justinian. In 1816 Niebuhr examined a palimpsest in the cathedral library at Verona, containing 251 pages, of which one detached and undefaced leaf of two pages had been described and partly published by Scipio Maffei 60 years before, with a conjecture that it was part of a compendium of Justinian's Institutes. With this exception the whole original manuscript had been washed and sometimes scratched out and overlaid with the epistles of St. Jerome, and 63 pages had been written over a second time; yet Niebuhr succeeded in restoring and deciphering a portion of it.

He communicated the results of his labors to Savigny, who published them, together with a learned note suggesting that the ancient text of the parchment was the lost Institutes of Gaius. The royal academy of Berlin in 1817 sent two accomplished civilians, Go-schen and Bethmann-Hollweg, to Verona, who, after incredible labor in deciphering the characters on the parchment, succeeded in making a transcript of the original writing, with the exception of three leaves and a few scattered passages which were illegible. A comparison of the work with the quotations in the Digest, and its agreement with the Institutes of Justinian, confirmed Savigny's conjecture, and the discovery, by clearing up difficulties in the interpretation of ancient jurisprudence before regarded as hopeless, formed an era in the study of Roman law. Several editions of the text have been published, that of Goschen (improved by Lachmann) of 1842 being considered the best; and commentaries on detached portions by Van Assen, Heffler, Klenze, Bocking, and others, have appeared.

The text, with an English translation and commentary by Tomkins and Seniors, was published in London in 1869; a translation with notes, by Abdy and Walker, in 1870; and a translation and commentary, by Edward Poste, at Oxford in 1871.