A simple guide to musical modes. Most musicians know about major and minor scales and keys. One thing that Jazz musicians and those interested in music from pre-1600 have in common is that they both know about modes. Modes existed before major and minor scales and during the late 1500's, composers started to alter notes within a given mode by using accidentals known as musica ficta. The musica ficta then developed into major and minor scales around 1600. But what are these modes? How do they work and what are they called? If you want to know about modes and to be sure you know your Dorian from your Phrygian, then this music theory lesson is for you.