In ascending order of size, units were: company, regiment, brigade, division, corps. Theoretically, company strength was 100; regiment, 1,000; brigade, 4,000; and division, 12,000. Occasionally, more often in the Confederate army battalions of 2 to 10 companies were accepted into the ranks. In the Union army, the actual numbers, by the attrition of war, were only 40-50% of those figures by 1863; the percentage was higher in the Confederate army, thanks to its system of assigning recruits to existing regiments instead of creating new regiments.
In the Union armies the number of division in a corps varied from 2 to 4, though usually there were 3. In spring 1863, Maj. Gen. Joseph "Fighting Joe" Hooker ordered the army of the Potomac to wear Corps Badges, which led to designating divisions by badges and by flags in red, white, and blue, for 1st, 2nd, and 3rd, divisions, respectively; the few 4th divisions had green badges and flags; 5th divisions, orange. Without uniform badges and flags, the Confederates used a less complicated system. Though they began by numbering their divisions, in a short time divisions, as well as other army units, come to be known by their commanders' name.
Union division were commanded by brigadier or major generals, and the frontage of an average 1863 Union division, drawn up in double-rank line of battle with no skirmishers deployed, would have been just short of a mile. The Confederates were more logical: with rare exceptions, brigadiers commanded brigades and major generals led division, and these units were usually numerically superior to their Union counterparts. An extreme example: at one time Confederate Maj. Gen. Ambrose P. Hill's famous Light Division had 7 brigades, giving it a strength of about 17,000.