In this style of part writing (16th century counterpoint), we want each voice to be a strong melody. Certain errors make voices blend together and resolve incorrectly.
Part Writing Errors:
Parallel Perfect Fifths
Parrallel Perfect Octaves
Unacceptable Unequal Fifths
Similar Perfect Fifths or Octaves
Must be between the soprano and the bass, and require a leap of a third of more.
When the voices move in similar motion and create a P5 or P8
This is avoided by using stepwise motion in the soprano voice.
See 10a for the rules.
See 10b for part writing error checklist/document.
Always prioritize smooth voice leading (doesn’t sound the most interesting, but provides the least part writing traps/errors).
Primary Errors
These errors must be within the same two voices across both chords.
Most of these primary errors are symptoms of bad voice leading, bad spacing, and all those other good things. If you are writing with those things in mind, you will probably not have to worry about these errors.