Seen from a viewpoint in the western Sudan, the prime significance of our period is that it saw the slow change in its external orientation, from the north and north-east towards the west and the south. At the start it was a region centred upon the empire of Mali, high up the valley of the River Niger and close to the frontier dividing modern Mali and Guinea. The Mali empire in the middle of the thirteenth century was taking tribute from many peoples besides its own Mande-speaking ethnic core. Westwards, its area of patronage stretched for some 700 miles to the Atlantic seaboard between the Senegal and Gambia rivers, where the Wolof and the Fulbe recognised its paramountcy. Eastwards, its authority extended for a similar distance downriver to Timbuktu and Gao, the two great meeting-points of desert trails and river traffic, where the precious salt of the Sahara was exchanged for the cereal produce, the slave captives, the ivory and the gold of the western Sudan. In Mali, as in other African empires, the supreme ruler was essentially a paramount, a king of kings, the degree of whose authority varied greatly from one part of his dominions to another, according to the accessibility of each to the imperial armies and tax collectors.