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6.5.5 Decending Thirds Sequence, Ascending Thirds: How Third Progressions Work
Just as harmonic progressions with roots a fifth or a fourth apart span the same harmonic space, so harmonic progressions with roots either a third (e.g., Am – C) or sixth (C – Am) apart span the same harmonic space.
By convention, these are both called third progressions. And again, unlike the situation with melodic intervals, by convention, there’s no such thing as a harmonic interval called a sixth (a “sixth progression”).
- A third progression up (or ascending thirds sequence) means counting forward by letter-name from the first chord root to the next one in the progression. So Am – C is a third progression up.
- A third progression down (or descending thirds sequence) means counting backward by letter-name from the first chord root to the next one in the progression. So C – Am is a third progression down.
Even though the same two chords are used, the sequence of chords matters. The progression Am – C sounds different from the progression C – Am. Just as a fifth progression up sounds different from a fifth progression down, so a third progression up sounds different from a third progression down.