I ran across a set of interesting images under "octave clef" at http://www.dolmetsch.com/musictheory14.htm . Since this qualified as my "learn something new every day," I just wondered if other folks have seen these particular clef markings? I admit to never having seen one (and in fact own a couple cello sonatas which are marked w/ a standard Treble clef but are intended to be played sub*va).
Music for guitar is written in the octave down G clef, so any guitarist that reads notation sees it all the time. Tenors (voices) use it too. Other instruments use other octave clefs. I was told piccolo and soprano recorder use the octave up G clef.
Most mixed voices choir scores I’ve encountered are written with a G clef for women, a F clef for bass and a sub octave G clef for tenor. Complete with the little 8 below the clef.
So, yes, I encounter them on a regular basis.
Pavarotti wasn't called the "king of the high C's" because he could actually hit a high C in treble clef! The note he could hit was a treble C. However, it would be written the same as high C in treble clef (two ledger lines above the top) because tenor writing either has the sub octave G clef or it is understood to be there.
Violas almost always use Alto clef. (That's the so-called "movable clef", the sort of K or B-shaped clef that moves around to be Alto or Tenor or Soprano or whatever clef.) Alto clef marks the middle line as middle C. Cellos and bassons usually use bass clef, but will switch to tenor clef to write higher note passages without using too many ledger lines.
I've never seen anything but treble and bass clef in piano music either. Most pianists can automatically read any note with up to three ledger lines. Once it gets much lower or higher than that, the music transposes up or down an octave and uses the 8va symbol.
In the cello repertoire, this happens only very occasionally. I think the main place it might throw you for a loop would be Dvorak chamber music, if you turn the page and suddenly you're looking at one phrase set in treble-clef-take-it-down-an-octave. I don't remember whether the little symbol below the clef was omitted or not, but I rather think so.
This will probably only cause a hiccup if you're sightreading from a newly purchased copy. If you're working from a library copy or a borrowed copy, likely there will be a little piece of paper with the one phrase written out in tenor clef, neatly scotch-taped into the cello part.
I guess if you want to be ready, and be able to read such a phrase on the spot, you could have some fun playing cello and violin duets with yourself at home, with a recording device. But the main usefulness of that skill would be that you'd be able to demonstrate something to a violinist in a quartet rehearsal. Or you'd be able to fill in for a violinist who had to step out to use the bathroom or take a phone call.
The only one you're going to come across in current music is the octave-down-treble-clef for choir tenors. Yes, piccolo reads an octave down, guitar and bass read an octave up. But they don't bother with the special clefs. What YOU mustn't do is use an octave clef as a substitute for 8va lines, for any instrument. It won't be noticed or understood.