When is a C not a C? It’s not a riddle or a trick question. It’s
a musical principle. The answer is: a C is not a C when you’re
playing a transposing musical instrument. And it’s not the
instrument that transposes, unfortunately — some arranger or
the player himself must do that.

To understand what this means, you first have to understand the
definition of concert pitch. Pitch, of course, refers to the
frequency of sound. Concert pitch is the universally agreed upon
definition of the pitch of middle C (and subsequently, every
other note in an octave). From the dawn of written music to the
present day, no matter where on earth you are, middle C always
sounds the same. In other words, every musician in the world
understands and interprets middle C in the same way. This is the
simple definition of concert pitch.

Many instruments, therefore, are manufactured to concert pitch.
This means that when multiple musicians read “C” (or any other
note on a scale) on a musical score, they produce identical
sounds. Instruments that are manufactured in concert pitch
include the piano, some brass instruments such as the trombone
and baritone, some woodwinds such as the flute, oboe, and
bassoon.

Many other instruments are not made to produce concert pitch
sounds. These are referred to as transposing instruments. Many
woodwind instruments and some brass fall into this category.
When a transposing instrument player reads and plays “C” in a
musical score, it sounds different from a “C” played by a
non-transposing instrument.

Therefore, music that is written for a band consisting of both
transposing and non-transposing instruments must be adapted for
individual types of instruments. A typical marching band, for
example, may have the same score of music transposed into two,
three or more different keys.

The clarinet is an excellent example of this. It is a
transposing instrument, typically manufactured in B flat. If a B
flat clarinet and a piano played a “C” at the same time, the
sounds produced would not be the same. In order for a B flat
clarinet to play a note that matches a piano playing a “C,” it
would have to play one whole step higher. In other words, it
would have to play a “D.”

What if the two instruments had to play a song together? Music
for the piano would be written at concert pitch. The same music
would have to be transposed one key higher so that the clarinet
would produce the same sound. Though the two songs appear to be
in different keys, the sounds that each instrument would produce
will match in pitch.

Bb instruments include the Bb clarinet, the Bb bass clarinet,
the Bb tenor sax, the Bb trumpet. Music for these instruments
are written in a key which is a whole step higher than concert
pitch.

Eb instruments include the Eb clarinet, the Eb alto clarinet,
the Eb alto sax, the Eb baritone sax, and the Eb alto horn. Music
for this group of instruments needs to be written a minor 3rd
lower than concert pitch.

F instruments include the french horn and the English horn.
Music for these instruments must be written in a key that is a
perfect 4th lower than concert pitch.

It sounds complicated. You may even ask: why not just
manufacture the clarinet (or other transposing instruments) in
concert pitch? Though it seems this would solve some problems,
there are good reasons that certain instruments produce other
pitches. The B flat clarinet is preferred because it produces a
cleaner, more pleasant sound than a C soprano (concert pitch)
clarinet. In fact, the same is true of many transposing
instruments. So while using transposing instruments make more
work for the composer, in the end it’s all about the product.

Serious musicians are known for their keen sense of hearing.
Since only the best quality sound will do for them, composers
will keep writing scores with both types of instruments in mind.