If an instrument is "in B flat", for example, then this means that when the instrument plays music with no sharps or flats (i.e., in C) then the music will sound in B flat. To put it more simply, if an instrument "in B flat" plays a written C, it will sound as a B flat.

The interval of transposition is a major 2nd (the interval between B flat and C), and therefore if we want this instrument, in B flat, to play alongside other instruments that do not transpose (instruments "in C", in other words), then we will have to transpose the music for the B flat instrument up by a major 2nd.

Likewise, if we want to play music that has been written for a B flat instrument (such as a trumpet) on a non-transposing instrument (such as a piano), we will have to transpose it down by a major 2nd to hear the correct notes.

Up or down?

It can be confusing to remember which way to transpose: up or down?

A simple way to remember is to imagine the transposing instrument playing a C major scale that you've written out. This will sound the same as the major scale that the instrument is "in", e.g. it will sound like B flat major when played by an instrument "in B flat". Therefore by writing out a C major scale, you have effectively transposed a B flat major scale for an instrument "in B flat", by transposing it up a major 2nd (from B flat to C). If you want to know what note a C written for an instrument "in B flat" will sound as, it's the same as the name of the instrument: so B flat in this case.

Similarly, a French horn in F playing a written C major scale will sound as the scale of F major a perfect fifth below the C major scale you've written: so you transpose up a perfect 5th to write for a French horn in F. Therefore, if you want a French horn to play an F, you would write a C; if you want them to play a G, you write D, and so on. Conversely, to find the pitches that would sound from some music written for a French horn, you would transpose down by a perfect fifth: a written C sounds as F, and a written D sounds as G, and so on.

A C major scale...
...played on a trumpet in B flat sounds as B flat major...
...so to write for a trumpet in B flat, transpose UP a major 2nd!

Note that in the example above, we have transposed the key signature as well: it is very important to remember to do this.

But... why?

But why do some instruments use this system?

One important reason is to allow a player to use the same fingering to play a different size (and therefore differently pitched) instrument.

Therefore, a clarinet player can pick up a clarinet in B flat, in E flat, or in A (all common types of clarinet!) and play music using the same fingering for all instruments. For example, a soprano clarinet in E flat is much smaller and produces a higher sound than a standard B flat clarinet. If we didn't use a system of transposition, then to play a written-down C (which, not being transposed, is supposed to sound like a C) would require different fingering depending on the instrument, and the player would have to learn a whole new set of fingerings for each size of clarinet. Reading from transposed parts means that the same written note will sound differently on each size clarinet, so they only have to learn one set of fingerings.

The reason behind the French horn's transposition is similar, in that it's to do with the size of the instrument. In the past, before valves, the French horn could only play "in" one key (technically, they could only play notes in the harmonic series based on the fundamental pitch of the instrument). In order to play a different set of notes, the French horn player would physically increase or decrease the size of the instrument by adding or removing "crooks", or small sections of tubing, changing the key of the instrument from, for example, "in F" to "in D". This complicated system has been replaced by modern valves which mechanically alter the length of tubing, but for these historical reasons, we keep the most common "length" of French horn: it plays music transposed "in F" (written up a perfect 5th from sounding pitches).

Unfortunately for horn players, a lot of old editions of music have not been re-transposed into F, and so they commonly find themselves being expected to transpose at sight from another key into F. If you play the French horn, you better get good at transposition!

Quick Quiz

So you've got the hang of transposing instruments? This will be easy for you then:

Q. What is the sounding interval between a middle C as written for a clarinet in B flat, and middle C as written for a French horn in F?

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