The Complete Idiot''s Guide to Music Theory

Part 6:
Arranging
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Of course, you can save yourself a lot of time and trouble by just telling the
drummer how many measures there are in each section of your song and letting
him (or her) make up his (or her) own part. That’s particularly common when
you’re arranging for a rock or jazz band.
Transposition
Many instruments read one note and play another—at least compared to con-
cert pitch (the actual notes as played on a piano). For example, a trumpet reads
a C but sounds a B
. These so-called 
transposing instruments
need to have their
music transposed to a different key to play in the same concert key as all the
other instruments.
Let’s take the trumpet again. Because the trumpet always sounds a major second
lower than written, you need to write the trumpet part a major second (two half
steps) 
higher
than the pitch you actually want to hear. So if you want the trum-
pet to play a concert C, you have to write a D; the trumpet reads D, sounds C,
and everything is right with the world.
You learned about transposition back in Chapter 14, and you’ll need to apply
those skills here. That’s because there are a lot of instruments that don’t play in
concert key; all the following instruments need their parts transposed.
Most transposing instruments fall into three groups, and are named according
to how they relate to C:
◆
B
instruments, like the trumpet, sound a major second below their writ-
ten pitch. That is, they’re written a major second above the concert pitch.
So if one of these instruments plays a C, it comes out sounding as B
in
concert pitch.
◆
E
instruments, like the alto sax, sound a major sixth below (or a minor
third above) their written pitch. That is, they’re written a major sixth
above the concert pitch. So if one of these instruments plays a C, it comes
out sounding as E
in concert pitch.
◆
F instruments, such as the French horn, sound a perfect fifth below their
written pitch. That is, they’re written a perfect fifth above the concert
pitch. So if one of these instruments plays a C, it comes out sounding as F
in concert pitch.
The following table details which instruments fall into which group:
Transposing Instruments