If you ever played 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, i.e. they are transposing instruments. 

Transposing instruments are instruments that are notated in a different key than they sound. This means that when music is written for a transposing instrument, the written notes are different from the actual pitches that are played. For example, a saxophone is a transposing instrument because the music written for it is notated in the key of C, but the pitches that are actually played are transposed down a certain number of half steps (a measure of the distance between two pitches). Since the saxophone is in Bb, the notes that you hear are two half steps below the notes you read on the score. 

Transposing instruments are used in a variety of musical settings, including orchestras, bands, and ensembles. They are often used to play music written in keys that are not easily playable on non-transposing instruments, or to allow musicians to play music in a more comfortable range. Many wind and brass instruments, including the saxophone, trumpet, and trombone, are transposing instruments.

Transposition is when music is notated differently between instruments, yet they are audibly playing the same pitch. 

There are a few reasons why some instruments are made in different keys.

One reason is that different instruments have different ranges, and so to play the same music on two different instruments, the music may need to be transposed to a different key to fit within the range of each instrument. For example, a trumpet and a clarinet may both be asked to play the same melody, but the trumpet's range is higher than the clarinet's range, so the melody may need to be written in a different key for each instrument to make it playable.

Another reason is that some instruments are easier to play in certain keys due to the way they are constructed. For example, some wind instruments, such as the saxophone, are made with a series of keys that allow the player to produce different notes by pressing down on the keys with their fingers. These keys are arranged in a way that makes it easier to play in some keys than others, depending on the number of keys that need to be pressed down at the same time.

Let's take a closer 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 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:

Listen to the flute melody in 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:

Listen to the the Bb clarinet transposed!

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:

Listen to the clarinet transposed for concert pitch!

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.

An important thing to note here is that when we say that an instrument is in the key of C, that means that a C Major scale sounds like a C Major scale, an e minor scale sounds like an e minor scale, and so on. We never have to transpose instruments written in the key of C, even if we are not playing in the key of C. Similarly, when a Bb instrument plays a C Major scale, it sounds like a Bb Major scale, and when a Bb Major instrument plays an e minor scale, it sounds like a Db minor scale. So, we are always transposing up a M2, regardless of the key we are writing in. 

The idea of interval inversions comes into the picture here. Sure, you could just count the number of half steps down the transposed instrument is, and then count up the same number of half steps to transpose it, but this is a pretty tedious process.  Notice that Bb is also a minor 7th above C. What is the inverse? A Major 2nd! So, we need to transpose everything up a Major 2nd.

➡️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!

However, for your own musical knowledge, here’s a short list of common transposing instruments: 

Here is a list of common transposing instruments:

• Bb clarinet

• Eb alto saxophone

• F horn

• Bb trumpet

• Eb tuba

• Bb bass clarinet

• Eb baritone saxophone

• F baritone horn

• Bb euphonium

🦜 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?