clef change problem for viola

[RMK]

A far as we know, viola was always written in alto clef. The only instrument written in alto clef, actually. (probably thus it's name alto = viola in many languages)

I think you forgot about the Trombone (and not just the alto instrument). Plenty of examples in Mozart, Dvorak, etc.

[David Ward]

… … … The only instrument written in alto clef, actually.… … …

The alto trombone, a standard instrument of the classical and now actively revived as a modern (ie not just period) instrument is always written in the alto clef. As a consequence, many tenor trombone parts for, say, Schubert masses also employ the alto clef, as indeed happens in quite a lot of Russian music (eg Prokofiev). There is a collection of studies for trombone by Blazevich called Studies in Clefs which changes in unpredictable and seemingly irrational ways between alto and tenor clefs (as well as bass) to familiarize the would be orchestral trombonist with the three clefs he/she is liable to encounter. As a bass trombonist, I never had to play from a part in the alto clef in a professional orchestra, but I still worked through these studies (and others).

Once when asked to play with an amateur orchestra for a Schubert mass, I was given the second (ie tenor) trombone part to play from the alto clef. The regular first trombone, playing the alto trombone part on a tenor, already had everything copied into the bass clef and transposed down an octave when high. The bass trombone part was played on a medium bore tenor with F attachment, so my full-sized bass in the middle, often playing higher than the transposed alto part, made for some unusual voicing. However, it was all for the benefit of the chorus, and went well enough as far as they and their friends in the audience were concerned.

EDIT: I hadn't seen RMK's post above when I typed this (and then had to do something else before posting), but I'll leave mine as it is.

[MichelRE]

correction, trombone uses it as well.
though for the viola it's the de facto clef.

[MalteM]

A far as we know, viola was always written in alto clef. The only instrument written in alto clef, actually.

According to the Neue Bach-Ausgabe, Bach uses the alto clef for alto oboes like Taille or Oboe da caccia. Interestingly he has both instruments in BWV 80 but in this case Taille is alto clef and concert pitch but Oboe da caccia treble clef and transposing in F.

[MichelRE]

I'm not talking about instruments that are no longer regular parts of the orchestra.

[John Ruggero]

And then there were the days when keyboard players read in 5 clefs: soprano, alto, tenor, violin (or G-clef) and bass.

[NeeraWM]

I would then argue for a politically correct revival of mezzosoprano and baritone clefs! :-D

[OCTO]

The only instrument written in alto clef, actually.

Interestingly, Prokofiev notated the English horn in alto clef—not because he wanted to change history, but because he thought it looked better! However, this was only in the score, and he orchestrated everything in C.

[MichelRE]

I have a Samuel Barber score where all instruments that are even remotely in an alto range are in alto clef, for example, the horns.
But this is one single score, everything else of his is notated "normally".
maybe he was trying something out in that score. I presume the actual horn parts were in treble/bass clefs (I don't have access to an actual set of parts.)