The smallest fighting unit for the infantry during the Civil War was the company. Companies generally consisted of 100 men on paper but were seldom up to strength due to casualties and illnesses. The staff of a company comprised of a Captain, who commanded, a 1st Lieutenant, 2nd Lieutenant and two Sergeants, and several Corporals. While the company was the smallest unit, it would at times be split up into platoons, sections, and squads, but not for extended periods of time and rarely, if ever, acting as independent commands.
Infantry companies were banded together with other companies to form battalions or regiments. Generally, there were eight companies to a battalion and ten companies to a regiment (the Union sometimes used twelve) and were designated with letters from the alphabet such as "A", "B", "C", "D", etc. (The letter "J" was not used because it looked too much like the letter "I".) Companies often carried the name of the individual or individuals who organized the company or for the place from where they came. For example, Company "M" of the second Florida Infantry Regiment was also known as the Howell Guards or the Dennison Guard from Ohio, organized at Camp Dennison. The staff of a regiment included a Colonel who commanded, a Lieutenant Colonel, Major, 1st Lieutenant (acted as an Adjutant), a surgeon, Assistant Surgeon, Quartermaster, Commissary Officer, and a Sergeant Major. The regiment was the primary fighting force for both the Union and the Confederacy.
Regiments were usually grouped together with other regiments to form a brigade. Brigades were commanded by a Brigadier General, and usually, but not always, regiments from the same state were brigaded together. Confederate Brigades were generally known by the name of the Brigadier General who commanded it, such as Wilcox's Brigade. Wilcox's Brigade was commanded by Cadmus M. Wilcox and was comprised of the 8th, 9th, 10th, 11th, and 14th Alabama Infantry Regiments. Union Brigades were usually numbered.
When several brigades were grouped together, they formed a division. Major Generals led divisions with Confederate divisions being named for the general who commanded it, such as Cheatham's Division in the Army of Tennessee during the Atlanta Campaign. Union divisions were numbered with Roman numerals.
When several divisions were organized together, they formed a corps. A corps was commanded by a Lieutenant General and could operate independently or operate as part of the larger army, which was their usual role. Like other large Confederate units, Confederate corps were named for their commander, such as Longstreet's Corps. Union Corps were numbered as were the rest of military organizations, except for Armies.
Armies were the largest of all the fighting units during the Civil War and were composed of corps, divisions, brigades, and regiments and included artillery, cavalry, signal corps, and various other units. A Lieutenant General or a General generally led armies.
An army organization may look something like this: