I am reading Mann's translation of Fux's the art of counterpoint and there are some musical notation bits that I, uneducated barbarian that I am, do not understand. these are all from the first chapter or two
On many of the examples, there is something that looks like the roman numer two (II), twisted 90 degrees sideways (=) (it has serifs so it looks like a II rather than an '=' sign), in the last bar. It doesnt seem to be a rest, since he puts it at different pitch locations.
Also, he uses a cleff notation I dont understand. He uses the treble clef, but BEFORE it, and separated from it by a thick bar line, he uses an alto clef. The cantus firmus and the counterpoint example have both treble clefs in the same position - but the alto clef before one of them them can be raised or lowered (ie the counterpoint might have an alto clef that is centred on e rather than on c, but the respectice cantus firmus has one centred on c).
there is sometimes also a small 8 subscript after a treble clef - does this mean that it sounds an octave lower than written?
The twisted "II" is a double-whole - or square note - which has a value of 2 whole notes. For instance, in 4/2 time signature, it lasts for the whole bar. It is common in counterpoint "alla breve" type of writing. It is a leftover of the old medieval notation.
The C-clef before the treble clef is called "prefatory": it simply indicates which clef was used in the original, provided that now standard (treble or bass) clefs are used for convenience. It has no consequence on the music.
The "8" added to a clef (bass or treble) is much more important: it indeed indicates an octave transposition! The most common is the Tenor treble clef, meant to be performed an octave lower than written.