The Complete Idiot''s Guide to Music Theory

Part 1:
Tones
If you think other musicians might be confused about whether a note has
reverted back to normal, it’s okay to use a 
courtesy
sharp, flat, or natural sign.
(This is a sign placed within parentheses.) This reminds the reader that the note
has reverted back to its normal state. You don’t have to use courtesy signs like
this, but when the music is complicated, it can be quite helpful.
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A courtesy accidental reminds musicians that a changed note has reverted back to normal.
Changing Keys
Some long pieces of music don’t always use the same key throughout the entire
piece. In fact, some short pop songs change keys midway through. It’s allowed.
When you change keys in the middle of a song, it’s called 
modulating
to another
key. You can modulate to any key, although the most common modulations are
up a half step (from E Major to F Major, for example), or up a fourth or fifth
(from E Major to either A Major or B Major, for example).
When you want to change keys, you indicate this by inserting a new key signa-
ture in the first measure of the new key. It’s as simple as that, as you can see in
the following figure. (Note that some composers and arrangers also insert a
double bar whenever there’s a key change.)
The half-step modula-
tion is most common
in twentieth-century
popular music, and
can add a “lift” to the end
of a pop song. The fourth
or fifth modulation is more
common in classical music
of the seventeenth through
the nineteenth centuries.
Tip
To change keys, insert a new key signature.
The only complicated key change is when you’re changing to the key of C—
which has no sharps or flats. You indicate this by using natural signs to cancel
out the previous sharps or flats, like this:
If you want, you can alert musicians to a key change by placing the
appropriate sharps and flats at the very end of the last staff of the old
key—as well as with a new key signature in the following measure. This
approach is entirely optional; it’s perfectly acceptable to signal the key
change with a single key signature in the first measure of the new key.
Tip

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Chapter 4:
Major and Minor Keys
How to change to the key of C.
The Least You Need to Know
◆
You use key signatures to indicate what scale your music is based on.
◆
The sharps and flats in a key signature are automatically applied through-
out the entire song.
◆
To indicate notes outside the current key, use accidentals—sharps, flats,
and natural signs.
◆
To change the key in the middle of a piece of music, insert a new key sig-
nature.
Exercises