Theoretical key
For example, if a piece in E major were to modulate up a fifth to B major, which is next to it on the circle and on the same side of the circle, the notation is clear and simple; an extra sharp is added to make five sharps. If it was modulated again, it would become F major, written with six sharps. The next modulation, to C has seven sharps. But the next modulation would be to a theoretical key of G which would need an eighth sharp- for the previous F note to become F in the key signature. To avoid theoretical keys, the new section would usually be written in the enharmonically equivalent key signature of A major.