In reply to Fraser:
> The plus is a full grade harder than one without. There are no "half grades" in French terminology.
In which case is F4 to F4+ a full grade? I think you'd say yes. Which means the naming is not definitive, simply nominal (e.g, each step is one grade harder and we've just chosen to call them 4,4+,5,5+,6a,6a+... etc.)
Which is fine, but if the names are arbitrary and do not define a grade, what is 4a, or 5a+? Undefined. If the list is defined by each step being one grade harder than the last, we can't start adding more in.
If the names were definitive we could work out what they mean: 6a+ is one grade harder than 6a so 4a+ must be one grade harder than 4a but this is incompatible with the statement that 4 to 5 is two grades.
Practicall, we have to assume that the three labels 4a,4b and 4c cover 4 and 4+ which is really only a range of two grades. Likewise 5a, 5a+...5c+ are six labels for a range of only two grades but 6a,6a+... are six labels for a range of six grades. This is starting to make adjectival grades sounds positively layman-friendly!
I appreciate that grading is so subjective this is all a waste of our time practically speaking, but I enjoy it as an intellectual exercise.