In the Middle Ages emperors and popes built a balanced dual system of power-sharing. On the basis of this assumption, Heike Mierau outlines her history of the relationship between empire and papacy from Constantine the Great to the Reformation. In her study the traditional, at least subliminal, view of a constant rivalry between the two universal powers is replaced by a model of cooperation: the medieval world was bound by a bipolar order. To substantiate her thesis, the author approaches her ambitious work in two steps. In the first part (pp. 15–161) she describes the common history of emperors and popes in chronological order, emphasising significant stages like the interaction of Constantine and Sylvester i, the investiture controversy, the deposition of Frederick ii or the long-lasting conflict between Louis iv called ‘the Bavarian’ and the Avignon papacy. This short historical review is followed by a second part, systematically outlining the phenomenon of a bipolar world order (pp. 163–262). The legal framework of the respective authorities, the political theory of the offices, the symbolism of contemporary concepts of world order as well as the fleshing out of individual ranges of action are illustrated. The volume is completed by a rich appendix of references, a list of popes and emperors, a bibliography and an index of names. A closer proofreading might have prevented the odd orthographic mistake and incidental linguistic lapses.

The attempt to deal with 1,200 years of papal and imperial history in part i provokes detailed criticism. It is, for example, unlikely that in 1075 Gregory's contemporaries did not venture to ‘go public’ with the Dictatus papae (p. 71) while labelling the document simultaneously as ‘a secret policy paper of the pope’ (p. 72). Canossa is deemed a reestablishment of a ‘twofold leadership’ (p. 73), the influence of the princes on the concordat of Worms in 1122 is ignored and the valuation of the election of Felix v by the Council of Basle in 1439 turns out to be problematic. In principle, the question arises whether the late antique and early medieval Roman episcopate can be equated with the high- and late medieval papacy, whose capacities to enter into dialogue or conflict with the emperors were completely different from those of its historical ancestors.

The actual rationale for this new concept is developed in part ii of the book. Mierau indicates that canon law was not an exclusive rule for the clergy. It was the constitutional framework for a divided but cooperating world of laity and clergy which was symbolised by the complementarity of the spiritual and the secular ‘sword’. Conflicts such as the hierarchic interpretation of the two swords' theory in favour of the pope are continuously qualified as being exceptional, which indicates, as a corollary, a stable basic order. The author relentlessly assembles references through the centuries that apparently sustain the image of a consensus-based bipartite guiding of the world. In this argumentation she does not distinguish between small notes on the topic and complete drafts of world orders. For example, Hugh of St Victor describes the universitas Christiana as a joint assembly of laity and clergy, which represent separate orders with distinct leaders. All in all, Mierau clearly recognises that both universal powers were in an unstable position and that periods of real equality were scarce (p. 207).

It is not possible to agree with all points of the explanation. Thus, in most medieval cartularies the deeds of emperors and popes have a prominent position but not in such a way as to symbolise the equal status of both issuers (p. 243) – how could this be achieved at all in a serial compilation? Since the thirteenth century most registers place the pope and clerical issuers first, followed by secular authorities. It is also questionable whether a parallelism of pope and emperor, for example when sitting on a throne together, can be interpreted as evidence for a divided ordo mundi or just as a ceremonial coequality.

Heike Mierau challenges some of our much-loved views on medieval order. The errors and density of the first part of the book make for an unconvincing read. The second part, however, presents itself as an essay rich in ideas about the relationship between popes and emperors, but it would have found a more appropriate place in a scholarly journal. Finally, the excessive use of the word ‘bipolar’ in a variety of contexts (climax: ‘bipolar disapproval’, p. 127) fosters the suspicion that the fundamental idea of cooperation has been less documented than is suggested in this book.