José Rodríguez Alvira
Let's take a musical example. Here we show the first two measures for clarinets in Bb of Debussy's Nuages.
As you see, the composer specifies in the score that the music is written for clarinets in Bb.
With this information we know that when the clarinets play these notes the real key signature is that of D major / B minor. That's the key signature used by Debussy on the other non-transposing instruments. Using the tenor clef we are able to read the real notes: