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I have seen in some music theory books triads are called common triads or sometimes I see common chords. What does common exactly point to? Does common chord mean a complete chord? in that case do we have incomplete triads? if not what does "common" exactly mean for a triad?

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    When you say music theory books are these books related to a particular instrument (piano, guitar) or just music theory in general. To me a common chord in piano would be different than a common chord in guitar because some chords are very difficult to play on guitar but are easy on piano. Fingering on guitar varies greatly between chords where on piano the same basic hand position can play all of the major chords, all of the minor chords, all of the etc. chords just by moving the basic hand position to a different place on the keyboard. – Rockin Cowboy Aug 19 '18 at 16:33
  • @RockinCowboy, in this context, the meaning of common is not "commonplace." – jdjazz Aug 19 '18 at 18:13
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Your book may use 'common' to simply mean 'basic'. Maybe as a synonym for 'Primary triads', I IV and V, the 'three chord trick'. But 'common' is also used to describe chords that appear in two different keys. F major chord is 'common' to C major and Bb major. It can therefore be used as a stepping-stone when modulating from C major to Bb major.

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    Excellent point about potential different meanings of "common chord" Laurence. We don't know which "music theory books" the OP is referring to. – Rockin Cowboy Aug 19 '18 at 16:27
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    That book uses common triads when it's not a dim or aug triad. But still some online theory texts use common chord. – Arash Aug 24 '18 at 6:37
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There are three possible meanings. One is more colloquial (the one mentioned by Tim) and another is more technical (pointed out by Laurence).

The third meaning--which I think is most likely--hasn't been mentioned yet, so I'll put them all together in one place. In general, a "common chord" could mean:

  1. a commonly-used chord (given that the context is a music theory book, this usage seems unlikely)
  2. a chord that is shared between two different keys (Laurence gives a great example)
  3. a chord with a defined chord quality (major, minor, etc.)

To understand #3, we must look at the word chord itself. In one sense, a chord is any two notes played simultaneously. But in another sense, a chord is three notes with a defined chord quality (major, minor, etc.). This second term is also called a definite chord, a common chord, or even triad. The modification common triad is a bit redundant, since any triad will have a defined chord quality. However, the author is likely simply calling attention to the fact that any triad has a defined chord quality.

I don't know of a term for the "opposite" of this, but we might call it an indefinite chord: for example, a chord with only two notes does not have a defined quality (major, minor, etc.).

  • Whilst convinced that two notes constitute a chord, two notes can produce a major or minor sound. Trouble is, those two which produce a minor sound could be construed to be part of a dominant seventh chord ! Do you agree that a common chord triad must have 'stacked thirds' to comply? And something like a sus chord - still a three note chord - isn't called a triad? – Tim Aug 20 '18 at 8:07
  • @Tim, yes! Everything you've written is exactly as I understand it too! – jdjazz Aug 21 '18 at 0:58
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Common here means most used. A triad consists of three notes (tri) and the common ones use root, third and fifth. In C, it's C E G; in Cm it's C Eb G. Another way to look at it is a pile of thirds - C>E is a third, and E>G is too.

There are other three note chords, such as sus chords, as in C sus 4, where the 3rd is replaced by a 4th. Not so common, and not 'stacked thirds'.

Can't imagine what an 'incomplete triad' might be! Possibly what guitarists call a power chord (5th)..?!

EDIT: Checking in my 'bible', the following is stated: triad= 3 note chord with root, third and fifth. Common chord = triad with perfect fifth. Major common chord = common chord with maj.3rd. Minor common chord = common chord with min.3rd. Aug.triad = 1, M3,#5, Dim. triad = 1, M3, b5.Those last two don't particularly apply to the question, but are still triads. No mention of 'common triads'...

  • In one text the writer puts common triads against the dim and aug triads. Refering to your bible common triad must have a perfect fifth, so aug and dim can not be a common triad, and because aug and dim are not used as much as others maybe common means "usual" or "ordinary" triads, that's how I understood so far. – Arash Aug 24 '18 at 6:31
  • Let's have the publication that quotes this, and we'll do a bit more research. – Tim Aug 24 '18 at 6:35
  • The book "Harmony for computer musicians" by Micheal Hewitt, uses common triads frequently. About common chords, I've seen it mostly on online texts, I don't remember a special book to refer. – Arash Aug 24 '18 at 6:44

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