A study of the stem family system in a group of villages in Haute-Provence during the eighteenth and nineteenth centuries reveals the existence of a high degree of isonymy (same-namedness). The sources employed were marriage contracts and parish registers permitting genealogical reconstitution. Does isonymy always signify a marriage between close relatives bearing the same name? In fact, although the spouses might share a common ancestor, that ancestor might date back many generations. The relatively frequent occurrence of marriages among homonymous spouses should not be interpreted as absolute proof of a union between close relatives. Indeed, heteronymic alliances, even exogamic ones, can hide a marriage between first cousins, if consanguinity is transmitted by the mothers.