The Culper Spy Ring, formed during the Revolutionary War, was the beginning of espionage in America.  Black petticoats hung on wash lines were used as signals.  

www.spymuseum.com provides an excellent description:

The Culper Spy Ring was established in 1778 by Benjamin Tallmadge under the orders of General George Washington. The ring was tasked with the mission of spying on the British Army and reporting on troop movements, positions, fortifications and plans in the New York area. The ring continued to operate until the end of the war in 1783.

In 1776, American spy Nathan Hale was hung after being caught by the British. Washington therefore determined to provide greater support to the Culper spy ring. The ring would use elaborate codes and aliases as well as dead drops and invisible ink in the course of their activities. Tallmadge (who went by the code name John Bolton) tapped two men for the task, Abraham Woodhull, a farmer from Setauket, New York and Robert Townsend, a merchant from Manhattan (Woodhull’s code name was Samuel Culper, Sr. and Townsend’s was Samuel Culper, Jr.).

Tavern keeper Austin Roe served as a courier and Caleb Brewster took the information on his ferry boat across the Long Island Sound where it was taken to General Washington. Anna Strong, the wife of a Long Island Patriot judge, would use her laundry as a way of signaling times and locations for the spies to meet. She would hang a black petticoat on the clothesline to indicate that Brewster was in town and available to ferry messages. She would then hang between one and six handkerchiefs on the line next to the petticoat in order to indicate the particular spot in which Brewster could be found.

Another unidentified member of the ring was referred to as 355. 355 was believed to have passed along information garnered from Major John Andre and Benedict Arnold and she was believed to have been arrested and taken to the HMS Jersey where she was questioned and would later die after giving birth to a child.

The manner in which the ring operated was ingenious according to the conventions and limitations of the day. It was operated in such secrecy that even General Washington did not know the identity of many of the key players. Townsend ran a dry goods store and also was the society reporter of a local American newspaper. His job as a reporter gave him access to British soldiers and functions without arousing suspicion. Roe would often drop by Townsend store and purchase goods as well as dropping off a special order from a Mr. Bolton.

After taking his goods he would leave and Townsend would go to an adjoining building and read the “request” and write a response. When Roe would come back to the adjoining building, Townsend would stuff his answers in with Roe’s goods. Roe would then ride by horseback more than 100 miles where he would leave the message in a dead drop box located in the middle of a field belonging to Woodhull. Woodhull would evaluate the information (sometimes adding to it) and then look across the bay at Strong’s signals to determine Brewster’s hiding place. Later that evening, Woodhull would find Brewster and give him the information. Brewster would then row across the water to Fairfield, Connecticut where he would meet with Tallmadge to deliver the messages, which would then be forwarded to Washington. The Culper Ring operated under constant pressure from Washington to obtain information but operated from even more pressure not to get caught by the British. Despite this, the ring is believed to have been the most successful on either side of the Revolutionary War. Of note, the group is believed to have played a central part in discovering the treachery of Benedict Arnold and to have learned of the British plans to ambush a unit of the French army arriving in Rhode Island (the consequences of which could have been devastating to the French and American newly-formed alliance).


- Found at www.spymuseum.com.