They could only be written (as they are yet) in one clef—namely, the F clef.
In addition to this Guido is generally credited with the introduction of the F clef.
The F clef requires three strokes, owing to the dots, each of which takes one to itself.
When the G clef is used the staff is usually referred to as the treble staff, and when the F clef is used, as the bass staff.
Whatever line the F clef is on is F, however many or few lines may be above or below it.