The palio of Siena, Italy, is a bareback horse race held in the city’s central square twice a year over the summer months. Over the centuries, this phenomenon has attracted the attention of scholars and visitors alike — perhaps more so than the 17 contrade, the city’s territorial communities that participate in the event. Our aim is twofold. First, we provide a primer on the contemporary contrada system. Second, we describe systematically the relationships between contrada pairs, which can be classified into one of three types: alliance, neutral relationship, and rivalry. To this end, we draw on published materials and on recent field-based research, including a cross-contrada survey of over 500 contrada members, a psychological item measuring the attitude of members towards the contrade, and the analysis of GPS data from an annual ritual. We find that membership size varies from a few hundred to several thousand individuals, a majority of whom are members from birth with familial and social ties within the contrada. We also find patterned variation with respect to relationship type in the attitude of contrada members, as well as in ritualized behaviour. We highlight the importance of the contrada phenomenon and the impact of group relationship on perceptions and behaviour.