I have a copy of Bach's BWV 1020 for flute and Bb clarinet + BC. Is the Bflat clarinet a transposing instrument? In other words, if I play the clarinet notes as written on the staff with a normal C flute, will those notes be OK for the harmony with the flute? (in fact a quick try I did not sound quite good...)
Since the Bb clarinet is a whole step lower than the Concert flute in C, you would have to play the Bb clarinet part a whole step higher to have the same concert pitch.
Same theory holds with Alto Flutes in G. When a concert flute in C tunes to an A, the Alto flute fingers and plays a D (a 4th lower) which will match the concert A since the Alto flute is a 4th lower.
Correction, the clarinet part would be written a whole step higher to be in concert pitch. Therefore to play your duet, the clarinet part should be played a whole step lower to match the flute part and BC.
Sorry for the confusion.
With the Alto Flute, it tunes to a D which is a 4th higher than A=440 since it is a 4th lower in concert pitch.
I was confused myself at first.
But I'm looking at a Beethoven Trio that I have for two flutes and Clarinet. Flutes are playing in the key of D. Bb Clarinet is in the key of E. Therefore, if one were to play the Bb Clarinet part on a C flute, they would need to transpose down a whole step.
And since there is an optional Alto flute part it is in the key of G which is a 4th higher than the key of D (since the Alto is a 4th lower than a C flute).