The great majority of the Army of the Potomac was made up of volunteer regiments furnished by the individual states. But there was a solid core of the United States Regular Army in all three branches of service. Over 6,000 Regulars served at Gettysburg, a larger contingent than all but two of the Northern and three Southern states. Parts of 13 infantry regiments, 4 cavalry regiments and 17 artillery batteries of U.S. Regulars were at the Battle of Gettysburg. They suffered over 1,200 casualties.
Monuments at Gettysburg to the regiments and batteries of the United States Regulars can be easily identified. All take the form of a rough hewn rectangle of stone with its top coming to a point, with a tablet and U.S. seal on the front.
The practice in the Army of the Potomac had been to keep the Regulars together as a solid, dependable reserve to back up the potentially unreliable state volunteers. But by mid 1863 many volunteer regiments had more combat experience than the Regulars. All of the Regular infantry regiments except one were still concentrated in two brigades in the Fifth Corps. The other infantry regiment served at Army Headquarters as Provost Guard. All four Regular cavalry regiments at Gettysburg were brigaded together in the Reserve Brigade of the Cavalry Corps.
The exception to this practice was the artillery. Five Regular batteries were brigaded together in the Artillery Reserve. But the other twelve batteries were distributed to the artillery brigades of the army. In each brigade one Regular battery served with three or more Volunteer batteries, providing a model and instructor for the Volunteers. This was probably a major factor in the high quality of the Union artillery.