If the original melody is in D Major, and the transposed instrument is a made up contra-alto instrument in Bbb (double flat), what key is the transposed istrument written in ?
I wonder, can you treat an instrument that is Bbb as an A instrument, and the transposed key would be F (is there any reason I couldn't do that ?) ?
A transposed instrument in the key of A is notated a minor third above concert-pitch and you would move three places to the left in the circle of fifths from the concert-pitch key to find the notated key. So, if you were to transpose the concert-pitch melody from D Major, and notate it for an instrument in A, you would notate the transposed score in the key of F.
A Bb instrument is notated a whole step above concert pitch (or the sounding pitch), and you can find the transposed key signature by moving two places to the right in the circle of 5ths from wherever the concert key is written. So, for a Bb instrument, notation for a concert-pitch melody written in D Major would be in E Major.
One last question : Does "contra" always refer to the sounding pitch being in some kind of octave transposition ? The transposed key for the Bbb instrument is supposed to be written in alto clef, which makes me think it should be written in a way that fits into the staff comfortably.
Anyway, your thoughts are appreciated

. Thanks in advance !