Some kinds of parallel motion must not occur except under appropriate harmonic circumstances because they destroy the four part texture: octaves, unisons, perfect fifths, and parallel dissonant intervals. Parallel perfect fourths are acceptable, but only in the upper three voices.
FIGURE 4.5: Objectionable Parallel Motion
When two parts move in similar motion to a unison, perfect fifth, or perfect octave, that movement is called Hidden or Covered. Hidden unisons are forbidden. Hidden perfect fifths and perfect octaves may occur at any time except between the Soprano and Bass voices at a change of chord with the Soprano moving by disjunct motion.
FIGURE 4.6: Objectionable Hidden Motion
Parallels by contrary motion occur when two voices move by contrary motion from one harmonic interval to the same harmonic interval. Parallel perfect octaves and parallel perfect fifths by contrary motion must not occur. An octave to a unison has the same effect.
FIGURE 4.7: Parallels by Contrary Motion
Crossed voices occur between any two adjacent voices where the lower voice has a pitch higher than the upper voice. Crossed voices are not normally permitted.
FIGURE 4.8: Crossed
Overlapping voices occur in the movement of any two voices when: a lower voice moves to a pitch higher than the previous pitch in an upper voice, or an upper voice moves to a pitch lower than the previous pitch in a lower voice. Overlapping voices are not normally permitted.
FIGURE 4.9: Overlapping Voices