Thus the king of Cashel was king over all the kingdoms of Munster; the under-kings owed him tribute and service in war, and he had certain obligations to them.[60] The king of Connaught and the king of Laigin held the same position in regard to the kings of those provinces, and the King of Tara exercised similar overlordship over the kings of Meath. But the king of Tara was also overlord of all the kings of Ireland, and his superior position was designated by the title “Árd-rí,” High King. The kings of Cashel, Connaught, and Laigin are usually described as provincial kings. For the island was regarded as consisting of five provinces or “fifths.” Connaught, Mumen, and “Ultonia” corresponded, with some minor differences, to Connaught, Munster, and Ulster of the modern map; while Leinster represents the two remaining fifths, Laigin in the south and Meath in the north. But it does not appear that in historical times there was any king who held the same position in the province of Ulster which the king of Cashel held in Munster. The northern province consisted of three large kingdoms, which seem to have been wholly independent, Aileach, Oriel, and Ulaid.[61] The kings of these territories were all alike overlords of under-kings; they were all alike subject to the High King; but they were as independent of one another as they were of the king of Connaught. The king of Ulaid was not under the king of Aileach, as the king of Thomond or the king of Ossory was under the king of Cashel.[62] [Sidenote: SYSTEM OF IRISH KINGDOMS] Ireland then was organised, theoretically, in an ascending scale of kings and over-kings. There was the High King at the head of all. Below him were six over-kings, the king of Cashel, the king of Connaught, the king of Laigin, the king of Aileach, the king of Ulaid, and the king of Oriel. Below these were the tribal kings, but in some cases there were intermediate grades, kings who were overlords of several small territories. For example, several of the small kingdoms in north Munster formed an intermediate group, the kingdom of Thomond. It is clear that this system must have grown up by degrees through conquest, and one remarkable practice illustrates its origin. It was the habit of the over-kings to take hostages from the under-kings, as a surety for the fulfilment of their obligations. This was such an important feature of the political system that a house for the custody of hostages was an almost indispensable addition to a royal palace. The “mound of the hostages” is still shown at Tara.[63] But though the general theory of the system is clear, it would be difficult to say how far it was a reality at any particular period, or how far the elaborate scheme of obligations and counter-obligations, binding on the kings of all ranks, was intended to be enforced. The ceaseless warfare which marks the annals of Ireland suggests that these bonds were a cause of trouble rather than a source of union.