Many people are confused by the concept of keys, and don't really understand
how keys work. As a result, you may hear some really silly things, like that a
piece is in "D dorian in the key of C major." (There's no such thing.) Here's a
really easy way to grasp this "key" theory concept:
Most of the music we listen to (with the possible exception of some 20th century
classical and electronic music) is tonal. This means that there is one tone (or pitch)
to which all the others maintain specific relationships. (Keys only exist in tonal music,)
This concept (the idea of a main pitch with relationships to all others) is called tonality.
Now, there are many different sorts of relationships that other notes can take with the
main tone (or tonic). Each set of relationships has unique characteristics, but
characteristics that can be duplicated regardless of which pitch we choose for
our tonic. This concept is called modality, and it is from this that we derive our
concept of scale. Historically, we most commonly use the major, the three minor
and the seven church mode modalities.
So, back to our key: When you combine a tonality (choose a tonic) with
a modality (choose a scale) that unique pairing is a key. So we can be in
the key of D (tonality) Dorian(modality) or C (tonality) major( modality), but
not both. (The confusion arises because both keys use the same key
signature in standard notation.)
I hope I have unlocked this very important concept for you.
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or use the contact form on this site if you have any further questions
about this (or any other) theory topic.