Until 1721, all 17 contrade were permitted to race, but the crowded conditions that resulted led to various accidents, and it was decided to reduce the number of participants to 10.
The latter are determined as follows :
- the 7 contrade which did not participate in the Palio of the previous year run "by right" in the following one (this ensures that each contrada will not remain more than a year without participating in the race),
- 3 additional contrade are drawn by lot among the 10 which participated in the Palio of the previous year.
Thus, whereas the names of the 7 contrade to run by right have been known for a whole year, the unveiling of the 3 additional ones is a moment of general excitement and great suspense, which draws to the Campo a large crowd of expectant contradaioli : not only those of the ten contrade that may hope to be among the three added, but also those of the seven guaranteed to run, who do not wish to see their rivals or enemies (if not already participating by right) among the happy three selected to join the race.
The ceremony is held within the Palazzo Pubblico, in the presence of the mayor and the captains of all the contrade. One after another, the names of three contrade are drawn from an urn and their respective flags are displayed on the façade of the Palazzo Pubblico at the first-floor level, next to the seven already there.
The seven remaining contrade, whose names have not been drawn and who will therefore not participate in the race (but will nevertheless take part in the historic parade before the race and are, besides, guaranteed to run the next Palio), have their own flags displayed on the front of the Palazzo Pubblico at the second-floor level, above those of the ten contestants.