ABSTRACT
There were important changes in tactical structures over the period from the 6th to the 11th century, and again from the later 11th to the 12th centuries and beyond. Units of the middle of the 6th century varied considerably in their regimental organisation. The older legions and auxiliary forces continued to exist through the 3rd and 4th centuries, divided into alae of cavalry and cohortes of infantry, nominally of 500 and 1,000 men respectively; although under Constantine I (324–37), new infantry units called auxilia often replaced these cohortes. Newer legions, numbering 1,000–1,500, had also been created during the 2nd and 3rd centuries, and this number seems also to have applied to the original legions by the 4th century. Apart from these were units called vexillationes, originally detachments from various units formed for a particular reason during the period c. 150–250, which had been turned into permanent units in their own right. This term, vexillation, was applied in the 4th century to most of the new cavalry units recruited at that time. Although some of these technical differences survived into the 6th century, the general term for most units was by then the word numerus or its Greek equivalent, arithmos or tagma, which simply meant ‘unit’ or ‘number’ (of soldiers).
