During this period, a military formation of around 5,000 men was known as a legion (Latin: "legio"). However, in contrast to later legionary formations of exclusively heavy infantry, the legions of the early and middle Republic consisted of both light and heavy infantry. The term "manipular legion", a legion based on units called maniples, is therefore used to contrast the later "cohortal" legion of the Empire that was based around a system of "cohort" units. The manipular legion was based partially upon social class and partially upon age and military experience. It therefore represents a theoretical compromise between the earlier class-based army and the class-free armies of later years. In practice, even slaves were at one time pressed into the army of the Republic out of necessity. Normally a single legion was raised each year, but in 366 BC two legions were raised in a single year for the first time.