There are no exceptions to these practices under any conditions:
- Certain types of parallel motion interfere either with the individuality of parts or with the forward momentum of the voice leading. By using similar motion there’s the risk of introducing consecutive unisons, octaves, and/or fifths. Avoid parallel perfect octaves (P8ths), parallel perfect fifths (P5ths), and parallel unisons (P1s). Successive perfect intervals containing the same pitches are not considered parallel.
- Never double the leading tone of the scale.
- Do not write pitches outside the range of a particular voice. Keep all four voices within their ranges at all times.
- Avoid the melodic augmented second (A2) and fourth (A4) in all voices.
Observe these practices carefully unless particular situations permit no other alternative:
- Avoid crossing voices. Keep voices in proper order (from highest to lowest): soprano, alto, tenor, bass. On rare occasions, crossing of voices is justified if it improves voice leading.
- Spacing between adjacent voices should not exceed an octave in the three upper voices. The spacing between bass and tenor voices can be of any reasonable interval (never greater than two octaves).
- Do not overlap two adjacent voices more than a whole step. An overlap occurs between two chords when one voice moves above or below the previous pitch of an adjacent voice. You may employ overlaps of a half or whole step if it improves voice leading.
- Do not move in the same direction to perfect intervals in the two outer voices (soprano and bass). Some theorists think that such motion, especially in outer voices, creates the effect of parallel perfect intervals.
- Unequal fifths, P5ths to d5ths or vice versa, are found in chorale harmonizations and may be used sparingly. The progression viio6 to I, under certain circumstances, requires the use of unequal fifths.
- Melodic augmented seconds and fourths are almost never found in choral literature of the eighteenth century.
- The melodic descending d5th appears sometimes in bass voices, but rarely in the soprano.
- The d4th is a diatonic interval in the harmonic minor scale (from the third down to seventh scale degrees) and may be written in isolated situations.
- The leading tone should progress upward to the tonic when it is in an outer voice (soprano or bass).