If you ever played an in a musical ensemble, you'll likely know that not every instrument plays in the same key. For example, if you told a tuba, clarinet, trombone and alto sax to play a C, they'd all play different pitches. Say what?
That's because each of these instruments are pitched in a different key, so the same pitch is notated differently between all of them.
Thus, we must transpose!
Transposition is when music is notated differently between instruments, yet they are audibly playing the same pitch.
To answer why, let's take a look at the saxophone family. An alto saxophone is an Eb instrument and a tenor saxophone is a Bb instrument. This makes it easier for a musician to switch between the two and keep the same fingerings. Both instruments use the same fingerings for the note Ab, but because they are different sizes, a different note will come out. So therefore their music is written in different keys so they can audibly play the same pitch, while the notes might be written in as a G and a C.
It's important that before you analyze a piece of music that uses instruments in different keys, you transpose them all to concert pitch. That way you'll analyze the pitches that are audibly being played, not each instrument's translation of that note.
But how do you actually transpose? Let's take a simple flute melody in the key of C major:
Let's transpose this example for a Bb clarinet. But before we do, let's see what the clarinet would sound like without transposing.
The notes in red show the pitches that you would actually hear:
As you can see, the clarinet sounds a major 2nd below the flute, which is in the concert key of C. This means that the clarinet is in the key of Bb because Bb is a major second (a whole-step) below C. So in order for these two instruments to play the same note, we must transpose the clarinet into the key that is a whole step above the key of C. That means the clarinet must play in the key of D major!
Here is the clarinet part notated in black, which in concert pitch will sound like the notes in red:
Notice how the red notes, the notes you hear, are now in the key of C just like the original flute part! And the written clarinet part is now in the key of D major, a whole-step above the original key.
➡️For the AP® test, you will not need to memorize the keys in which every instrument will be played. But you will need to know how to transpose when told the key and level of transposition of an instrument or voice part. For example, a prompt involving transposition will say "Horn in F, sounding a perfect 5th below the notated pitch". And you, being the great student that you are, will know to transpose that part up a perfect 5th!
🦜 Polly wants a progress tracker: Say you have a trombone part in the key of C and you want to play it on the alto sax which is in the key of Eb, sounding a minor third above the trombone part. What key do you need to transpose the alto sax to?
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