Writing Transposing Instrument Music (I)

If we would like a flute and a clarinet in Bb to play in unison the following melody:

We must transpose the clarinet part an ascending major second so that the desired notes are heard. A clarinet player could do it at first sight if he reads the music using the alto clef and changes the key signature to that of D major (ascending major second). A composer or an arranger could also use this clef and key to transpose and write the part for a clarinet player.