[1] When called to enact legislation or make decisions, such as on guilt or war, citizens were in the historical period always divided into voting blocks.
There were three kinds of voting blocks – curiae, centuriae, and tribus – giving rise, respectively, to the curiate, centuriate, and tribal assemblies.
A comitia was an assembly summoned to make a decision, about whom should be elected, whether a law should be passed, war and peace, or guilt.
It was organised on the basis of curiae and is said to have been the main legislative and electoral assembly of the regal and early republican periods.
It was made up of 193 centuries (Latin: centuriae) which were apportioned to Roman citizens by wealth and age, hugely overweighting the old and wealthy.
There is scholarly disagreement as to the extent to which the comitia centuriata facilitated competitive elections, even within its de facto restrictive electorate.
The traditional view is that Roman elections were largely unrepresentative of the population as a whole and dominated by the wealthy through social connections.
In the standard conception of the classical republican constitution, it was essentially identical to the tribal assembly except that patricians were excluded and it was presided over mainly be plebeian tribunes.
The main legislative assembly in the republic, it also elected the plebeian magistrates (tribunes and aediles) and heard some judicial matters.
It is the modern convention to refer to an assembly of the people, organised by tribe and under the presidency of a plebeian tribune, as a concilium plebis.
The main difference between the contio and other public assemblies in Rome, such as the comitia, is that the citizens who attended contiones were there to hear speeches and not to vote.
The contio merely served a communicative function, offering magistrates the opportunity to give the people a report of what had been decided during a senate meeting or to discuss a proposed legislative bill (rogatio) in front of the citizens to help them make up their mind before they had to vote on it in other assemblies.