Air on the G string (with score)
https://m.youtube.com/watch?v=E2j-frfK-yg#fauxfullscreen
A few hints to understand what is going on are the following:
The composition is in the key of D major. Ie all notes are natural except F which is F sharp.
Bach used step bass and at the same time bass moving up and down in octaves.
While the bass moves step by step other voices may remain constant creating for example prepared 7ths, 9ths etc.
Together with a 7th, 9th with respect to bass, the 3rd is added by another voice (part or melody).
For example at the beginning the violas and the violins play A D #F ie the chord D major (derived by the simple 1 3 5 rule with respect to bass D, ie D bass and D #F A).
Bass steps down the scale note by note while other parts remain A D #F. When bass becomes B the part playing A becomes B ie parts play again 1 3 5 wrt bass, the B minor 3rd chord.
Then bass keeps going step by step down the D major scale. Upper parts already play B D #F which is really looking ahead to the creation of the G B D #F chord when bass eventually becomes G. This fantastically sounding chord is therefore ingeniously prepared for the listener's brain. It is called G major 3rd major 7th, the name derived from the name of the intervals of the other parts with respect to bass.
Throughout the composition exquisite musical intervals such as 9ths with respect to bass are created in a prepared way for the human brain and another part or voice is then simultaneously playing the interval of 3rd with respect to bass.
See also:
Bach bass rules - Bach, Niedt
https://normanschmidt.net/scores/bachjs-general_bass_rules.pdf
Harmony - Piston
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