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Third can refer to:

  1. an interval (major or minor third)
  2. the pitch class that is one third away from the tonic
  3. the third scale degree

In case of the meaning 2., the pitch class that is one third away from the tonic is not always the third scale degree, right? In the pentatonic minor scale the "third" is actually the second scale degree as far as I know and understand it.

Am I right or wrong about this?

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    The problem here is you’re conflating the third with a third. 1. is a third. 2. and 3. are both the third (of a given key). Commented Oct 16, 2023 at 0:23
  • @ToddWilcox Alright, I got it. But the things I listed are correct in themselves or are they not? Commented Oct 16, 2023 at 6:15
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    I would argue, to @ToddWilcox 's point, that #2 and 3 are not proper terminology. Maybe a lazy shorthand, maybe ok if context has made it crystal clear that we're talking about scale members. But the little ^ thing exists for a reason! Commented Oct 16, 2023 at 14:23
  • @AndyBonner Thank you. But when refering to the pitch of a chord, for example, you'd say the triad consists of a root, third and pitch. Third and fifth refer to both an interval and the pitch class that the interval "ends on". If the root happens to be the tonic, then third and third scale degree would be the same or is this misleading? Commented Oct 16, 2023 at 18:54

3 Answers 3

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No.1 is correct — a third interval is C>E in some form or another, could be C♯>E, C>E♭, C♭>E♯, all of which are called 'thirds'.

Scale degrees only works for diatonic scales. The 3rd from C is an E of some sort again, but only diatonically.

Extrapolating the pentatonic idea, no. It won't work for that, the same as it won't work for notes from the chromatic scale.

2

A third is an interval.

The concept of a 'third' is very closely tied to the concept of a 7-note diatonic scale using all the notes ABCDEFG, each possibly sharpened or flattened (even multiply).
A third is the interval from one of the notes of this scale to the next-but-one (A->C, A->C#, B->D, Bb->D, Bb->Db, F->A etc.). Most thirds are major (4 semitones) or minor (3 ST), but others are possible:

  • diminished e.g. F#->Ab equivalent (in 12-tone equal temperament at least) to a second F#->G#,
  • augmented Gb->B equivalent (in 12-TET) to a fourth F#->B, and
  • theoretically even doubly augmented/diminished.

The third scale degree of a scale with more or less than seven notes can be anything. In a chromatic scale it's a major second, in a pentatonic scale it might be a perfect fourth

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In interval-speak, a third is the interval that encompasses three letter names. C to E, C to E♭, C to E♯ - all thirds because C, D, E are three letters. C to D♯ is a second, C to F♭ is a fourth. Same notes, different spellings. But three letter names=a third.

'Third' of course also has an everyday meaning.The third note of 'God Save the Queen' is A. The third word of the preceding sentence is 'course'.

In the context of 7-note scales, the third note of a scale will invariably be 'a third' above the tonic. (Can we concoct an exception? Can't see how, but go for it!) Not all scales have 7 notes. We can immediately think of chromatic, whole-tone and pentatonic scales where the third note may not be 'a third' above the tonic.

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