Lotharingia and East Francia: Marriage as a Political Instrument? – Kings and nobles arranged marriages for their daughters in order to form or strengthen po- litical alliances. Historical writers of the tenth century interpreted the relations of the Ottonian kings Henry I and Otto I with the Lotharingian dukes Giselbert and Conrad the Red in this way. Since both dukes later joined in rebellions against Otto I, the effectiveness of such marriage alliances in a medium- and long-term perspective is at least questionable. In approaching this subject, I not only consid- er narrative sources, but also official royal documents, which have so far been neglected for this context. Through these, we can analyze the kings’ own views – or at least those of their chanceries – since they provide a picture of relations between king and son-in-law that has not been influenced by hindsight. In this manner, we can examine the efficacy of marriage alliances as a political instru- ment.