The jurors for each trial were chosen from a large body of citizens available for jury duty for the period of one year. At the beginning of the year, each juror was given a bronze pinakion, a plaque that had his name, father's name, and deme (and therefore tribe) inscribed on it.
The pinakia were used in kleroteria, allotment machines that assigned jurors to the courts. The procedure worked as follows: On the day a trial was to be held, the potential juror would appear before the magistrate in charge of the allotment who was stationed at one of these machines. At the base of the kleroterion were ten baskets, one for each of the ten tribes. The pinakion would go into the appropriate tribal basket, which was labeled with the name of the juror's tribe. When it was time to allot jurors to courts, the magistrate would take the pinakia from the first tribal basket and put them into the first vertical row of slots in the machine, the pinakia from the second basket into the second row, and so on until he had placed all the pinakia into slots.
Along the side of the machine was a hollow bronze tube, with a funnel at the top and a crank at the bottom. Into the funnel the magistrate poured a mixture of white and black marbles, which would line up in the tube in random order. A turn of the crank at the bottom produced a single ball. If it was white, the ten citizens (one from each tribe) whose pinakia were set into the first horizontal row would be assigned to the jury for that day and would proceed at once to the court. If it was a black ball, all citizens whose pinakia were in that row were dismissed for the day. The procedure was repeated until a court was filled, selecting ten jurors with every white ball.
The machine assured absolutely random selection, both in the order in which the pinakia were placed in the kleroterion and in the order in which the balls appeared. There was no easy way to bribe an Athenian jury, made up of at least 201 men chosen immediately before the court sat. At the same time, the kleroterion chose one juror from each of the ten tribes with each white ball, so that there was equal tribal representation on every court. The machine could also be used to appoint a board of ten magistrates, in this case only one of the balls would be white. As much as any object left to us from antiquity, the kleroterion indicates the lengths to which the Athenians went in trying both to ensure equality and to forestall corruption in their governmental affairs.
Athenian jurors were paid, another democratic procedure designed to ensure that all could afford to serve. Small round lead tokens or symbola were issued to jurors who had been allotted to assure proper payment to the right individuals. Payment was made only at the end of the trial and only upon presentation of the symbolon. Numerous symbola have been found scattered over the Agora; they carry different devices and letters to indicate the court to which the juror was assigned.