A line from a spiritual song.

Back in the 19th century, when spiritual songs were popular, it was legal in some parts of the United States for people to own other people. The owned people would compose these spiritual songs, and sing them while they worked (for no pay). There were many such lines in spiritual songs:
  • Swing low sweet chariot
  • Goin' over Jordan, goin' over home
  • The trumpet sounds within in-a my soul, I ain't going to stay here
  • Steal away, steal away, steal away to Jesus
  • You got to move
  • (etc.)
These lines figuratively described the process of dying and finding contentment in the next world. They also became a way for owned people to speak in code about escaping from the bondage they lived under.

For instance, you might be one of these owned people and wish to escape from this bondage. To indicate this to another owned person who might be able to help you escape, you would sing, as you performed your labor for the peson who owned you:

Swing low, sweet chariot
Comin' for to carry me home...

To the person that owned you and his helpers, this would be seen as a good thing. Singing gave you a rhythm to work by as you perfomed your menial repetitive task, and improved your productivity. The owners were also good Christian folk who were glad to hear you singing such pious music, and it made them happy to know that the gospel seed they had planted had not fallen on infertile ground, but had taken root and grown.

To people who may be able to help you escape, you were indicating that you were willing to take a risk when the "chariot" (e.g., the train of people who would hide you as you made your way to a place where owning other people was not legal) came.

If you were fortunate enough to make a connection with this train, when the time came to escape, the person who was going to help you might indicate this by singing:

You may be high, you may be low,
You may be rich chil', you may be po',
But when the Lord get ready, you got to move....

Encoding messages in spiritual songs became a very effective means of communication and was employed by the Underground Railroad. Some spiritual songs took on very specific meanings, indicating escape to Canada (O Canaan), crossing the Ohio River (Roll Jordan Roll), and following the North Star (Follow the Drinking Gourd). Harriet Tubman especially enjoyed Swing Low. The practice was also described by the former slave Frederick Douglass.