Notes are notated in a system of 5 lines. A note can be on a line or
lie in the gap.
Thus, such a grading system can map eleven notes.
In music, of course, there are more than just eleven tones used, alone a piano has usually 88 keys. Therefore, a grading system can only ever contain a small section, represent a small range of possible grades.
In order to know which range a staff represents, it is marked with signs at the beginning
marked. These signs are called "clefs".
In our Western music, the most commonly used clefs are the G or treble clef and the F clef or bass clef.