type or another. Shields were four feet tall and two feet wide and made of elephant or ox hide. A heavy sword was the main armament for close combat. The Yoruba and their neighbours used triple barbed javelins which could be thrown accurately from about 30 paces. Structure The Oyo Empire, like many empires before it, used both local and tributary forces to expand its domains. The structure of the Oyo military prior to its imperial period was simple and closer aligned to the central government in metropolitan Oyo. This may have been adequate in the 14th century when Oyo controlled only its heartland, but to make and maintain conquest farther away, the structure underwent several changes. The Eso Oyo maintained a semi-standing army of specialist cavalry soldiers called the Eso, the Esho or, formally, the Eso of Ikoyi. These were 70 junior war chiefs who were nominated by the Oyo Mesi and confirmed by the Alaafin of Oyo. The Eso were appointed for their military skill without regard to heritage, although de facto dynasties of Eso were also known to exist. The Eso were led by the Are-Ona-Kakanfo, and were famous for living by a warrior code that was comparable to the Latin dictum infra dignitatem. Are Ona Kakanfo After Oyo's return from exile, the post of Are-Ona-Kakanfo was established as the supreme military commander. He was required to live in a frontier province of great importance to keep an eye on the enemy and to keep him from usurping the throne. During Oyo's imperial period, the Are-Ona-Kakanfo personally commanded the army in the field on all campaigns. Metropolitan Army Since the Are-Ona-Kakanfo could not reside near the capital, arrangements had to be made for the latter's protection in case of emergency. Forces inside metropolitan Oyo were commanded by the Bashorun, leading member of the Oyo Mesi. As stated earlier, Metropolitan Oyo was divided into six provinces divided evenly by a river. Provincial forces were thus grouped into two armies, under the Onikoyi and the Okere for the east and west side of the river respectively. Lesser war chiefs were known as Balogun, a title carried on by the soldiers of Oyo's successor state, Ibadan. Tributary Army Tributary leaders and provincial governors were responsible for collecting tribute and contributing troops under local generalship to the imperial army in times of emergency. Occasionally, tributary leaders would be ordered to attack