The_Complete_Idiot%27s_Guide_To_Music_Theory

Accidents Will Happen
When you assign a key signature to a piece of music, it’s assumed that all the
following notes will correspond to that particular key. How, then, do you indicate
notes that fall outside that key?
First, it should be noted that you can play outside a key. For example, it’s okay
to play the occasional B natural when you’re in the key of F, which normally has
a B-flat. No one will arrest you for it—in fact, certain types of music regularly
employ nonscale notes.
When you decide to write a note that isn’t within the current key, you have to
manually indicate the change in the music—by using sharp, flat, or natural
signs. When musicians see the inserted sharp, flat, or natural, they know to play
the note as written, rather than as indicated by the music’s key signature.
These “outside the key” notes are called accidentals or chromatic notes; they’re
quite common.
For example, let’s say a piece of music is in the key of F, which has only one flat
(B-flat). You want your melody to include an E-flat, which isn’t in the key. So
when you get to that note, you insert a flat sign before the E to indicate an
E-flat. It’s as simple as that.
Use accidentals to indicate notes outside the current key signature.
The same theory would apply if you want to include a B natural in the same
piece, instead of the expected B-flat. If you simply insert a natural sign before
the B, you’ve accomplished your mission.
When you change a note with an accidental, that accidental applies until the
end of the current measure. At the start of the next measure, it’s assumed that all
notes revert to what they should be, given the current key. So if you flat an E in
measure one of an F Major melody, the first E you write in measure two will be
assumed to be natural; not flatted.
The one exception to this rule occurs when you tie a note from the end of one
measure to the beginning of the next. The accidental carries over—thanks to the
tie—to that first note in the second measure, as you can see in the following example.
(Ties are explained in Chapter 5.) Note that the accidental doesn’t apply to
any subsequent notes in the second measure; it applies only to the tied note.
Accidentals apply to all notes tied over a measure.
Chapter 4: Major and Minor Keys
Note
Jazz and blues music often
add flatted thirds and sevenths
within the designated
major key, which
give these styles their
unique sound.
Warning
An accidental
applies only from that
point in the measure to the
end of the measure. It
doesn’t affect those notes
in the measure before the
accidental appears.
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