Blues is a form of primitive, repetitive music, utilising what is known as blue notes. Its precise origins unknown, the blues music today is derived from the crude Negro work songs sung in the cultural backwaters of southern US plantations during the late 19th century. These songs incorporated hollers, howls, shouts, and chants to a simple rhythm. The term blues comes from the phrase 'blue devils' meaning depression or melancholy. In music theory, the so-called 'blue note' is a lowered-fifth and a tritone with the root note. The tritone appeared in Western music for hundreds of years before the blues, and was banned in churches. Its dark and often unsettling sound, it was once called the ‘devil’s interval’ and compositions involving these notes were considered the work of the devil.