The great majority of the Army of the Potomac was made up of volunteer regiments furnished by the states, but there was a core of United States Army Regulars 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 batteries of U.S. Regulars were at the Battle of Gettysburg. They suffered over 1,200 casualties.
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. By mid 1863 many volunteer regiments had more combat experience than the Regulars, but all of the Regular infantry regiments except one were still concentrated in two brigades in the Fifth Corps, with the other regiment serving at Army Headquarters as Provost Guard. All four Regular cavalry regiments were brigaded together in the Reserve Brigade of the Cavalry Corps.
The exception was the artillery. While five Regular batteries were brigaded together in the Artillery Reserve, the other twelve had been 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.