Harmony interval: Definition: Harmony interval refers to two notes played at the same time. Writing: When writing a harmony interval, the two notes should be aligned up and down, but the bass should be on the left, the treble should be on the right, and the two notes should be close together. Modeling: Friends who study music are no strangers. That is: copy the original things and sing or hum the sounds you hear. It can also be that the teacher plays a melody at random on the piano, and the students hum the melody they hear, which is called "model singing".