The most basic infantry building block was the infantry regiment (referred to as a “battalion” in Hardee’s drill manual, which was used by both sides). In both North and South the infantry regiments were organised on similar lines.
A regiment typically comprised ten companies, each numbering 100 men, supported by staff, drummers and officers. However, the rigours of warfare would summarily lead to a reduction in strength by way of battle, disease and desertion, and a regiment's numbers could vary wildly.
The states of the north had a tendency to allow regiments to fall below an effective level, to a point where a number of such regiments would be forged into a brand-new formation. This sometimes had a detrimental effect as units lost their identities and their associated fighting spirit.
The most common identifier for a regiment was a number and the State in which they were raised – for example, the “15th Alabama” or the “4th Connecticut”.