NOTES ON THE ROMAN CONSTITUTION
The Senate or Council of Elders was a body of 300 with ten members from each of thirty voting districts (curiae), which in turn were based on the original three tribes. The senators, or fathers of families, represented the richest and most powerful clans. At one time, as a much smaller body, they had advised the king. Under the Republic they advised the magistrates who consulted them, but in virtue of their corporate experience their influence in every area of government was far weightier than that implies.
The comitia curiata or Assembly of Voting Districts, originally consisting wholly of patricians, ratified the appointment of a new king. It met at the king's behest to pledge loyalty in war or to endorse a death-sentence. It also had some more minor functions, such as witnessing wills and adoptions. Its importance dwindled with the passage of time.
The comitia centuriata or Assembly of Centuries, which was originally summoned by the king, met in the Campus Martius (Field of Mars). Its organization and procedure are described on pp. 47-8, 189-90. The function of the body was to enact laws; to elect consuls, praetors, and censors; to vote in capital cases (i.e. those involving death or exile); and to declare war and peace.
The comitia tributa or Assembly of Tribes, representing the whole population, was convened by a consul or praetor. It elected quaestors and curule aediles. As it met within the city and was more democratic than the Assembly of Centuries, it gradually increased in importance. Its measures were apparently subject to the approval of Senate until 339.
The concilium plebis or Council of the Plebs, like the comitia tributa was organized by tribes, but was not attended by patricians. It was presided over by a tribune and it elected tribunes and plebeian aediles. It also issued resolutions (plebiscita), which after 287 had the status of laws, and heard trials of non-capital offences. The Council drew closer in function to the Assembly of Tribes (which patricians often failed to attend), and frequently the two bodies are not distinguished in our sources.