In what cases do you double chord tones and why ? What are the rules pertaining to doubling chord tones ? I know about doubling chord tones when using power chords by in what other cases would you double them?
In the study of the Bach Chorales, which is where most Traditional Theory focuses for the extrapolation of so-called rules, the most common reason for doubling a tone was the fact that a triad has only three notes and in order to cover it with four part harmony, you HAVE to double one of the notes, usually the root if possible. OK to double the fifth, but never the third according to the 'rules.'
In fact Bach sometimes even tripled the root, and omitted the fifth altogether. Having a note appear in more than one instrument is NOT considered doubling, according to Traditional Theory, nor is reinforcing a line by doubling it at the octave, particularly if the line is the bass line. (Full Orchestration would be almost impossible otherwise.) The 'rules' only apply to four-part vocal harmony, what we now call SATB.
We double chord tones all the time, and I don't think you're going to find a single theoretical reason that tells you when you should, and when you shouldn't. At least, none that aren't very specific to a genre or style.
If you step back and think about it, lots of common chords only have three distinct tones anyway. If you have a whole band playing, you're going to run out of notes really quickly if you don't double them. Or triple them. Or quadruple them. You get the picture. In fact, you might say that it's very unusual to not double a chord tone, if you're playing with more than a few instruments. Even a single guitar or piano will generally double at least one tone.
Even more generally, we don't even consider doublings when we name a chord. The chord is defined by unique tones, and the fact that you play one more than once is pretty much immaterial.
This is somewhat of a sidetrack, but when we talk about theory, be careful not to overanalyse things. There are plenty of things in music that can be given names and described with complicated rules (with even more complicated exceptions). However, that generally adds very little value to our understanding, ability to play better, or enjoyment. Don't hear me saying that theory is useless; it's very important. But you may not be able to find rules and reasons for everything.
I think that it is okay to double just about anything if there is a good reason for doing so. I suspect one could find many examples in classical music of the 20th century though I haven't researched this detail before.
The rule for avoiding doubling mostly comes from contrapuntal concerns (which come up to a considerable extent with chorale writing exercises). If the goal is to preserve independent melodic lines that obey a certain rules of good form or "singability" then doubling the 3rd can be problematic. If a 3rd degree of a chord appears in a melody, it is often a leading tone to root of the next chord. Perfect example: the 3rd of V is the leading tone to the root of I. Any move by the melody other than to the root will be awkward or will call undue attention to itself (in a V-I cadence).
Other chord tones can usually be followed melodically by either ascending or descending tones. So, when doing Bach chorale exercises, where the point to a large extent is learning how to keep independent and singable lines going over harmony changes, one can preserve this independence by having the melody go in different directions from the two doubled tones.
But with two leading tones, it is very difficult to have them go in different directions. Both wish to follow their lead. Thus we get a parallel octave and a "fusing" of the two lines into one. For this reason, one avoids setting up the chord with two thirds and thus also avoids the "unsolvable" situation.
If there is no need to preserve or control independent melodic lines, then you can probably let color considerations rule the day in the construction of the chords.
As a general rule, the following applies to the doubling of notes when you have triads in 4-part-harmony.
You double the root when:
- You have a chord in root position
Chord in the first inversion (Except for diminished chords ie Super Tonic and Leading Tone chords)
Never when a chord is in the second inversion.
You double the third when:
- When you have a chord in the first Inversion (except when the third in question is the Leading Tone ie not in the Dominant chord.)
- Never when you have a chord in root position unless you have the following...
1) You have a minor chord.
2) It is the Dominant chord followed by Sub-Mediant (V - vi)
You double the fifth when:
- When you have a chord in root position
- When you have a chord in the first inversion, EXCEPT diminished chords
- ALWAYS when you have a chord in the second inversion.