During the first decade of the 1900s the War Department decided that a better understanding of the battle needed to be presented to the park visitor. Many monuments told the experiences of Union regiments and batteries, but other parts of the story were missing. As part of the same program that erected markers to Confederate headquarters and artillery batteries, a series of markers were placed that told the story of the higher level Union headquarters.
The Headquarters of the Army of the Potomac and each of its brigades, divisions and army corps is shown by a marker. These are located near where each headquarters operated and give a brief report of that unit's activities. A few headquarters have more than one marker.
Each type of headquarters has a unique monument design. You can learn how to identify these markers, as well as telling Union from Confederate, on this how-to page.