In 462 BC the tribune, Gaius Terentilius Harsa, proposed that a commission be appointed to gather up the laws and to publish them, so that the whole of the Roman people could read them. This proposal was bitterly opposed by the patricians and at first nothing came of it, there was continuing conflict between the two orders over the next few years and the patricians made various concessions to try and placate the unsettled plebian population, among other things the senate increased the number of tribunes from two to five, and then to ten, they gave the plebs some of the public land on the Avantine hill and limited the maximum fine that a magistrate could impose (the limit was set at two sheep and thirty oxen). This did not have the desired effect for the patricians as the plebians real desire was to have equal legal rights.

Finally in 452 BC compromise was reached and it was decided that in the following year no consuls or tribunes would be elected and instead a group of ten men, called decemvirs, would be appointed. These decemvirs main purpose was to write the laws of Rome down so that all people could read them, but they would also be granted entire control of the government during this period (why this was thought to be necessary is unknown).

The initial decemvirs were composed completely of patricians and supposedly they started by sending a commission of three men to Greece to consult the laws of Solon. There is doubt whether this actually happened because of two main reasons; It seems unlikely Rome would borrow the laws of another country to govern their own, especially as the laws that were published were quite different then Greek law at that time, the other issue was that the plebians were not questioning the fairness of the laws of Rome but instead simply wanted the existing laws to be written down so that they could know what they were and if they were properly administrated or not.

In 450 BC the First Decemvirate finished their task and ten brass 'tables' were erected in the forum which contained the laws of Rome. At the end of the year a Second Decemvirate was elected, this time composed of a mix of plebians and patricians,  to finish the codes of law and they added two more tables, bringing the total to twelve. This was a great event for the plebian order, not because there was any concessions to them in the laws in the twelve tables, but because it meant that the laws were now publicly known and the plebians (and the patricians) now knew their rights and the rights of every other person in Rome.

The second decemvirate exercised their power in a manner that can only be described as tyrannical, they seemed to actively oppress the plebians finally at the end of their year in office they refused to resign and continued to rule Rome under their leader Appius Claudius. The final straw appears to have been when Claudius attempted to use a legal trick to seize control of Virginia, the beautiful daughter of a plebian soldier, her father then killed her to save her from dishonour. This was the catalyst that caused the second secession of the plebians.

Virginia's father rushed back to his armies camp outside the city and urged the soldiers to resist the decemvirs, the army decided to secede from Rome again and with a large number of citizens of Rome they decided to set up a new city on the Sacred Mount again. Rome seemed on the verge of ruin with a severely weakened army and many citizens gone. The decemvirs were forced to resign and the old government was restored and two consuls, Valerius and Horatius, were elected, both of whom where friendly to the plebians.

The Twelve Tables had a provision prohibiting intermarriage between patricians and plebeians. This appeared in the eleventh table, the first of the two extra tables enacted by the second (illegal) college. The Roman tradition viewed this law as unfair, and it has been argued that the eleventh and twelfth tables were ascribed to a (made up) second college in order to separate the unfair enactment from the supposedly fair legislation of the ten tables enacted by the first college. The twelfth dealt with unobjectionable matter and was presumably added to the "unfair" legislation simply because the law considered unfair was in the eleventh. Thus , according to this view, the second college was concocted to explain away the unfairness of the one provision, while maintaining intact the reputation of the code as a whole. Although there is some logic to this position, there is no evidence that it was actually the provision about intermarriage that led to the idea that there were two colleges.

445 BCE || The Lex Canuleia

This law, a product of the continuing struggle between Patricians and Plebeians referred to as The Conflict of the Orders, allowed Patricians and Plebeians to intermarry.

http://www.forumromanum.org/history/morey08.html

http://www.ualberta.ca/~csmackay/CLASS_365/12.Tables.html