the Muslims. The Honorancie civitatis Papie commented that the Venetians were 'a nation that does not plough, sow or gather vintage', and by the end of the ninth century their control of trade had been established in large areas of Lombardy along the Po valley and the Adriatic coastline, where by the tenth century their power extended as far as Ancona. On the opposite shore, Istria and parts of northern Dalmatia became Venetian protectorates in return for help against the attacks of Slav pirates. The maintenance of safe passage in the Adriatic was always of prime importance to Venice, for her trade with the east, as well as other parts of Italy, had to pass along this route. In addition, the Dalmatian coasts and islands provided Venice with grain to feed her population and wood to construct the larger and cheaper vessels produced from the eleventh century onwards. The increasingly privileged position of the Venetian merchants in Constantinople can be traced in a series of Byzantine imperial documents (chrysobulls) issued in the tenth and eleventh centuries. In 992 Basil II agreed that Venetian goods should be admitted to Constantinople at lower tariffs than those levelled on their rivals, Jewish and Lombard traders, and merchants from Bari and Amalfi. About a century later the Venetians were assigned a trading quarter in Constantinople on a prime site on the shores of the Golden Horn and freedom from trade tolls and taxes in the Byzantine ports in the Aegean, though probably not in the Black Sea. The successful maritime expansion of Venice in the eastern Mediterranean has often been attributed to these treaties and it was certainly the case that they became bargaining counters in the political relationship between Byzantium and Venice, to be withdrawn and reinstated at the will of the empire. Much of the tension between Venice and the empire in the twelfth century arose from Byzantium's skilful manipulation of trading rights and their award in turn to Venice's rivals, especially Genoa and Pisa. In 1111 the Pisans were also allotted their own wharf in Constantinople and were guaranteed protection against the Venetians; by the 1160s they had sworn fealty to the emperor. The mutual hostility between the various trading groups erupted into a series of riots in the city in the 1160s
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