The falling thirds sequence is extremely common in Baroque music:
- In four parts:
Some four parts practice here
Notice that in four voices the middle voices also present an opportunity for imitation in the form of staggered diminution. The green voice (alto) alternates staying in place and moving down one step, while the blue voice (tenor) moves down by step and then stays in place.
Why is this sequence so useful and common?
- Vertical advantages
If we remove the middle voices, we are only left with alternating 3rds and 6ths descending down the scale. Vertically, this makes this sequence always consonant:
- Horizontal possibilities
Horizontally, this sequence offers repeated opportunities for imitation (stretto) because the melodic intervals always alternate +2 -3 in both the top and bottom voice, one position apart.
If we alternate the diminutions accordingly we can achieve a stretto:
This is possible with the third voice included:
- Modulatory possibilities
Both the motion ↓3 and the motion ↑2 in the bass can serve as a cadential motion. ↓3 would be an alto clausula and would be used in an evaded cadence and the ↑2 would be a discant clausula.
This offers many modulatory possibilities. A few examples follow: