It has been widely hypothesized that signals were often put into quilts. There were a bunch used; they (and what they signified) can be found here: http://www.osblackhistory.com/quiltcodes.php
A lantern in the window was sometimes used to identify a safe house: "The famous stationmaster John Rankin used this secret “sign” to signal that it was safe to cross the Ohio River to his home. However, this was not a common signal. If it had been, the slave catchers would have quickly learned of it, and used it to identify safe houses."
Songs with certain codes were also used in planning and coordinating the initial escape; see: http://ctl.du.edu/spirituals/freedom/coded.cfm
The stationmasters used signals on the Underground Railroad, including lanterns, overturned cups and tilted signs. (http://www.farmingtonhistoricalsociety-ct.org/fh_farmhist_pg4.html - no info on what those things meant though)
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Communication was key to the successful operation of the Underground Railroad Secrecy was vital. Safety was more important than quickness. Of necessity, both fugitive slaves and members of the Underground Railroad learned to code and decode hidden messages, to disguise signs and themselves to avoid capture or worse. There were code names for towns on the routes. Cleveland was called "Hope." Sandusky was called "Sunrise." There were code numbers for towns: Seville, Ohio, was code number 20.
"By Tuesday you shall receive a shipment of four large kegs of dark ale and one small one." was a message that four adults and one child would be arriving at the station.
There were signs. A quilt hanging on a clothesline with a house and a smoking chimney among its designs indicated a safe house.
There were signals. Each house had its own combination of knocks. For example, (three knocks), "Who’s there?" "A friend with friends."
There were no telephones and no access to telegraphs, so messages were sent by softly tapping stones together, by imitating the shrill call of the hoot owl, or by signaling with lights. Messages were also sent through song. Most slaves could not read and their masters had purposely kept them ignorant of geography. Spirituals were a safe way to express what the slave could not say openly. They could have double meanings and be used to send secret messages.
"Steal Away to Jesus" - double meaning; sung as an invitation to slaves to run away.
"Steal away, steal away to Jesus. Steal away, steal away home. I ain’t got long to stay here."
“Wade in the Water" - to warn an escaped slave that his master and his bloodhounds were on the trail. The only way to throw a bloodhound off a human scent was to get into the water.
"Wade in the water. Wade in the water. Children, God going to trouble the water.”
"Get on Board Little Children" - referred directly to the Underground Railroad.
"...Get on board, little children, There’s room for many more."
"Follow the Drinking Gourd” - taught to slaves in the South by a "conductor" named Peg Leg Joe, a free black and a former sailor who worked for hire at plantations as a handyman. The lyrics served as directions to Canada:
Chorus: Follow the drinking gourd! Follow the drinking gourd. "drinking gourd" - the Big Dipper, which points to the North Star. For the old man is awaiting for to carry you to freedom -Peg Leg Joe, waiting to help them at the Ohio River and into Canada. If you follow the drinking gourd.
When the sun comes back, "when the sun comes back" - meant slaves should travel in springtime. and the first quail calls, Follow the drinking gourd. For the old man is awaiting for to carry you to freedom Iffreedom if you follow the drinking gourd.
The riverbank makes a very good road, tThe dead trees will show you the way. Left foot, peg foot, traveling on, Follow the drinking gourd.
When the great big river meets the little river, “great big river” - the Ohio River. “The second river” - the Tennessee River. Follow the drinking gourd. For the old man is awaiting to carry you to freedom