Musical Cryptography? What is that?
It is a method in which the musical notes A through G are used to spell out words, abbreviations, or codes. This secret writing appears as a plot point in TV shows such as Outlander, the 18th-century time travel drama based in Scotland, and mystery novels like the Secret of the White Rose, which features characters who employ music ciphers as undetectable modes of espionage or communication.
The claim is that music is the perfect camouflage, because spies will never suspect music. When played, the music should sound so much like any other composition that musically trained listeners can be easily fooled, too.
You can read more in an article by Christina Ayele Djossa in the Atlas Obscura web site at http://bit.ly/2IuyncX.