Both Confederate and Union officers spoke freely in front of African American servants and left documents and maps where the servants could see them, assuming the slaves and servants would not understand what they were seeing and hearing. This was not always the case…In 1863, a runaway slave named Dabney and his wife joined Union forces led by General Joseph Hooker. Dabney worked in Hooker’s camp as a cook and servant. His wife left the camp after a short stay and returned to the Confederate side of the river. She quickly found employment as a laundress and personal servant for a woman where her duties included washing clothes for Confederate officers in Fredericksburg.
Shortly thereafter, Dabney began supplying Hooker with information about the Confederate troop movements. His reports were a wealth of information. He seemed to know what units were moving, where they had been and where they were going, the size of the units, and more. The information was extremely accurate.
No one could figure out how Dabney was getting this information. Despite the fact that he had a good knowledge of the surrounding countryside, he was always busy with his duties and was never seen leaving the camp. Finally, after much questioning, he told the officers his secret. He led them to a vantage point overlooking Fredericksburg and the Confederate side of the river. He pointed across the river to a small house on the outskirts of the city. He explained that the clothesline in the yard of the house was a type of telegraph. His wife, who washed and cooked for the Confederate officers there, had a system for hanging the laundry that told Dabney what the troops were doing.
“Well,” Dabney said to the Union officers,” that clothesline tells me in half an hour what goes on at Lee’s headquarters. You see my wife over there? She washes for the officers, cooks, and waits around, and as soon as she hears about any movement or anything going on she comes down and moves the clothes on that line so I can understand it in a minute. That there gray shirt is Longstreet; and when she takes it off [the line], it means he’s gone down about Richmond. That white shirt means Hill; and when she moves it up to the west end of the line, Hill’s corps has moved up stream. That red one is Stonewall Jackson. He’s down on the right now, and if he moves, she’ll move that red shirt.”
Jennifer J. Lawrence, Soap Suds Row: The Bold Lives of Army Laundresses, 1802-1876 (2016)