• Apr 12, 2020 - 19:49

I was told once that I should use thirds up or down to write a harmony.
I tried this but often I have to use both 3 semitones 3rds and
4 semitones 3rds to write the same harmony.
Is there a rule to know when to use a 3 semitones 3rd instead of a 4 semitone one, or vice versa?
Or is it done by personal choice?


'A" rule? No. Many? Sure.
Too many to learn here.
It depends on the chord, voice leading, passing tones, genre and volumes more.
Or, just write what you think sounds good.

Hi, a three semitones 3rd is a minor third, and a 4 semitones third is a major third. Each has its own different "feel" (sound) and which one you use depends on the type of sound you want.

In practice, the choice of which third to use normally depends on the actual key & chord in effect at the time. If it's a piece in D major, you will use notes from the D major scale mostly, but if an E major chord occurs somewhere, you'll probably want G# instead of G, etc.

Also, while thirds do often sound good, often it's best to also mix with sixths (particularly below the melody), and one way to choose between third and sixth is according to which note actually fits the chord better.

In reply to by underquark

The (now 50-year old) intro to Pinball Wizard is what is called a "pedal point passage", where the low F# is not supposed to harmonize with the chords above it, but potentially disagree with them, rhetorically holding constant while the above disagrees. Organists 400 years earlier did this with their feet, hence "pedal point", but it is the opposite of "harmony" -- it is one of the classical forms of dissonance,

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