While both the Do- and the Fa- clefs could theoretically be placed on any of the four lines of the staff, materials trying to give an explanation of chant notation usually claim or imply that only certain of these positions are ever used for each clef.
I've been noticing that these claims do not necessarily agree.
For example, this guide to Gregorian Chant Notation says "Do Clef - 3 positions (lines 2-3-4); Fa Clef - 2 positions (lines 2-3)", whereas the guide to gabc notation states "Clefs used in Gregorian chant: c2 c3 c4 f3 f4".
Where are these would-be authoritative claims coming from?
As for examples, it is easy to find chants with Do-clefs on lines 2-3-4 and Fa-clefs on line 3, e.g. in the Liber.
But does anyone know of any examples in the standard chant books of Do-clefs on line 1 or Fa-clefs on lines 1-2-4?
Looking especially for Fa-clefs, all that I've come across recently is the Henri du Mont Missa 6i toni, which puts its Fa-clefs on line 2.
The gabc documentation doesn't claim to be authoritative in regard to the vast variety of the chant repertoire. I'm not sure where you're getting that idea. In practice, Gregorio allows for either clef on any line.
Fa-clef on line 4 is very rare, but the link shows one example. Fa-clef on line 2 would be very unusual since it is effectively identical to Do-clef on line 4. Similarly, Fa-clef on line one is the same as Do-clef on line 3.
One of my fifth graders asked today whether there could be a Do clef on line 1. My answer is similar to Arthur's: It would be nearly the same as having a line 4 Do clef.
Since the purpose is to keep the notes centered on the staff, then this goal would be met just as easily, and by convention more "normally," by just keeping the Do clef on line 4.
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