Even if your band plays exclusively by ear, you won't last long if you don't understand Key theory. The first step is understanding there are twelve keys in Western music (plus three enharmonic keys that are sonically identical to one of the twelve). Each of these keys can be understood as a group of notes that share a common tonality. Can you say, "Plays well with others?" In order to have any idea which notes to play in a given piece of music, you need to know the diatonic scales and chords that make up that particular key. But how do we determine which scales and chords belong together? With the road map that follows, you will be well equipped to navigate without ending up in the wrong key.
Have you ever noticed that the black keys on a piano appear in alternating groups of two and three, respectively? That is because the formula for the diatonic scale has two whole step intervals, one half step interval, three whole step intervals, and one more half step interval. The black piano keys rest between these intervals in a C-diatonic scale. If you play only the white keys, you end up playing in C major. In the following diagram, notice the key of C has no sharps or flats.
In each key, the diatonic scale begins on the tonic (or first scale degree) of the key, then follows the same sequence of intervals. So if you wanted to determine the G major scale, you would start on G and add six more notes, being sure to add the single sharp in the key signature: G,A,B,C,D,E and F#. The key of D has two sharps, so it is written: D,E,F#,G,A,B and C#. As you proceed clockwise around the circle of fifths, each consecutive sharp key adds another sharp in a pre-determined order. Traveling the opposite direction from C, you get F major: F,G,A,Bb,C,D and E. The great thing is that when you build a diatonic scale, all the notes fit perfectly into the key in which you are playing.
Notice that between the sharp keys and the flat keys, there are three overlapping enharmonic keys that have the same notes but two different names.
Diatonic Chords in Each Key
If you build a 7th chord starting on each degree of the diatonic scale, you will arrive at the diatonic chords which belong in that key. These also have a pattern, starting with a major chord on the tonic and proceeding according to the following formula: Cmaj7, Dm7, Em7, Fmaj7, G7, Am, Bm7b5 (key of C).
Notice in the illustration, capital Roman numerals indicate major chords and lower-case numerals indicate minor chords. To arrive at the diatonic chords in a given key quickly, write out the scale, then add the chord designations after each note. D major would be: Dmaj7, Em7, F#m7, Gmaj7, A7, Bm7, C#m7b5
This can be very useful information. Knowing the chords that belong to the key you are in makes transcription and songwriting much easier. You already know what is going to fit. "But lots of songs have chords outside the diatonic structure," you say. "What about those cases?" The old adage, "You have to know the rules before you can break them," applies quite well. Sometimes great music is built upon the unexpected. But without the expected, what would that mean? Of course, once you have built a progression based upon diatonic chords, you can play the matching diatonic scale over it.
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