The quickest answer to your question is for you to take a look at the web
sites listed below. They explain shape note, and particularly Sacred Harp,
very well.
http://www.mcsr.olemiss.edu/~mudws/harp.html
http://www.mcsr.olemiss.edu/~mudws/resource/
http://fasola.org/introduction/note_shapes.html
http://www.paperlesshymnal.com/shapnote/shaped.htm

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