These rules apply for major and minor first inversion triads.
(1) Double the root or the fifth, preferably, of a first inversion triad. Only double the third if it provides the best possible voice leading.
(2) Don’t double the bass of a V6 chord, ever. The bass of V6 is the LT. Leading tones in the bass are always resolved.
(3) A change of position from V to V6 requires that the LT in the upper voice for the root position V change to a new chord tone. This change avoids doubling the LT. The upper voice change is usually from scale degree 7 to 2, so that the original LT can still resolve to 1. You want to resolve the original LT, as well as the new LT in the bass.
(4) When moving from root position to first inversion, or vice versa, the best voice leading is a voice exchange. (The bass and the soprano swap chord tones.)
(5) When a voice exchange is not possible, one voice must remain static.
(6) To avoid parallel octaves between root position and first inversion chords of different harmonies, approach and leave the doubled chord tone by oblique or contrary motion.
(7) Chordal figuration (chordal skips or changes of position) can be used to avoid parallel fifths and octaves.
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