In 1365, Murad received from the outside world the first acknowledgement of his commanding position as heir apparent of the Byzantine Empire. It was an overture from the flourishing republic of Ragusa, on the Dalmatian coast, for a treaty guaranteeing freedom of trade in the Ottoman dominions to the merchants of Ragusa. In return for unrestricted commercial privileges, the republic offered to pay a large sum annually, which the givers called a grant, but which was invariably accepted by the recipients as tribute.[275] However it may have been at the beginning, the grant soon became tribute, for after some years the existence of Ragusa depended upon purchasing the benevolence of the Ottoman sultans. As the helplessness of the Ragusans increased, the tribute became larger. If we except the convention between the Genoese and Orkhan, of whose provisions and character we know nothing, the Ragusan commercial treaty is the first of the long series of treaties by which European cities and nations purchased the right to trade in the Ottoman Empire and to sail the high seas. Since in most cases the Osmanlis pledged themselves to nothing except to refrain from robbing merchants or from preventing their trading, the gifts exacted were nothing less than blackmail. After the sea-power of the Osmanlis had been broken, the Barbary corsairs inherited the privileges of this system which had been started in so small a way by the Ragusans. Murad could not write. When the treaty with Ragusa was brought for his signature, he put his hand in the ink and made the impression of his fingers upon the paper. This is the origin of the _tughra_, which has ever since been the official signature of the house of Osman.[276] VI When Murad was settling himself in Adrianople, and laying plans for the conquest of Macedonia and Bulgaria, he was menaced by a new crusade. Despite its futile ending, or better, for that very reason, the expedition of Amadeo of Savoy in 1366 commands our attention. For it furnishes, as does the expedition of Admiral Boucicaut from Genoa in 1399, a striking illustration of how easily the growing Ottoman power might have been crushed by a resolute body of crusaders with a single aim, and of how impossible it was to secure that oneness of purpose, owing to the ingrained animosity of the East and West, of the Greek and Catholic Churches.