Digital Sound & Music: Concepts, Applications, & Science, Chapter 3, last updated 6/25/2013
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Like meter, the key of a musical composition is indicated at the beginning of each staff,
in the key signature. The key signature indicates which notes are to be played as sharps or flats.
The key signatures for all the major keys with sharps are given in Table 3.7. The key signatures
for all the major keys with flats are given in Table 3.8.
You may have noticed that the keys of F major and D minor have the same key signature,
each having one flat, B♭. So when you see the key signature for a musical composition and it
has one flat in it, how do you know if the composition is written in F major or D minor, and what
difference does it make? One difference lies in which note feels like the "home" note, the note to
which the music wants to return to be at rest. A composition in the key of F tends to end in a
chord that is rooted in F, as we’ll see in the section on intervals and chords. A second difference
is a subjective response to major and minor keys. Minor keys generally sound more somber, sad,
or serious while major keys are bright and happy to most listeners.
You can see in the tables below that each major key has a relative minor key, as indicated
by the keys being in the same row in the table and sharing a key signature. Given a major key k,
the relative minor is named by the note that is three semitones below k. For example, A is three
semitones below C, and thus A is the relative minor key with respect to C major. When keys are
described, if the words “major” and “minor” are not given, then the key is assumed to be a major
key.
You’ve seen how, given the key note, you can determine the key signature for both major
and minor keys. So how do you work in reverse? If you see the key signature, how can you tell
what key this represents? A trick for major keys is to name the note that is one semitone above
the last sharp in the key signature. For example, the last sharp in the second key signature in
Table 3.7 is F#. One semitone above F# is G, and thus this is the key of G major. For minor
keys, you name the note that is two semitones below the last sharp in the key signature. For the
first key signature in Table 3.7, the last sharp is F#. Two semitones below this is the note E.
Thus, this is also the key signature for E minor.
To determine a major key based on a key signature with flats, you name the note that is
five semitones below the last flat. In the key that has three flats, the last flat is A♭. Five
semitones below that is E♭, so this is the key of E♭major. (This turns out to be the next to last
flat in each key with at least two flats.) To determine a minor key based on a key signature with
flats, you name the note that is four semitones above the last flat. Thus, the key with three flats
is the key of C minor. (You could also have gotten this by going three semitones down from the
relative major key.)
Major key Sharps Minor key
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