Today let's discuss the principals behind connecting subdominant with the dominant chord. The opposite progression is generally not used in classical tonal music because there is a tendency to use chords with increasing intensity and tension. Because of the leading note (7th scale degree) the dominant chord has more tension than the subdominant chord. Therefore the progression D-S would be less natural. However, it sounds perfectly fine in pop music.
So anyway, here are the rules for connecting subdominant with the dominant. Note that these chords are spaced a major second apart so unlike the progression we discussed yesterday (S-T, T-S, D-T, and T-D) S and D have no common tones. Therefore we can only connect S with D using melodic method (moving all voices).
1) The bass moves stepwise upward from the root of one chord (4th scale degree) to the root of another chord (5th scale degree).
2) The upper three voices move stepwise downward (contrary to the bass).
Note that this type of voice leading (contrary motion with the bass) helps to avoid forbidden parallel octaves and fifths.
Try this exercise on paper and later on the instrument in various keys (major and minor). When playing in minor keys remember that the dominant chord has to be major - use raised 7th scale degree (just like in harmonic minor scale).
If you master T-D, D-T, S-T, T-S and S-D progressions from the lessons of yesterday and today in various keys, you will be equipped to harmonize many hymns just by using these chords (sometimes you will need to know a few more rules).
Although T, S, and D chords don't provide the many colours, variety and options that are needed for the mastery of hymn harmonization, they may well be your most easiest solutions (shortcuts) when it comes to hymn playing - liturgical organ playing would sound so much better for many organists if only they new these simple rules.
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