The first definitions of "civis" appeared in learned legal treatises of the 12th century and focused on the fiscal duties deriving from the belonging of a person to an urban community. In order to obtain a general contribution from the urban population, medieval Italian cities needed to get rid of the idea of subjection, traditionally implied by any form of taxation during the Middle Ages, and to insist on a new principle: the necessity of charging individuals for the common benefit. To what extent was such an innovative perspective influenced by new experiments in collecting payments carried out by the church during the 11th and 12th centuries? What was the role played by the rediscovery of Roman law for the development of a new interpretation of the tithe? To what extent can episcopal inventories be considered a model for lay appraisals developed by the communal Italian governments? This paper aims to answer these questions through a comparative analysis of the tax policies carried out by bishoprics and city-governments in both Italy and southern France, and it tries to establish a dialogue between practical tax experiments and civil and canon legal thought during the 12th century.