1 NO consecutive 5ths
When there are perfect 5ths in the same two parts one after the other, we call them “consecutive 5ths”.
2. NO consecutive octaves
When there are 2 octaves in the same two parts one after the other, we call them “consecutive octaves”
3. NO hidden (or “exposed”) consecutive
Hidden consecutive happen when:
The bass and soprano parts form a perfect 5th or octave AND
The 5th/8ve is approached by similar motion AND
The soprano part is approached by a leap (the interval between the two notes is wider than a 2nd).
4. NO dissonant leaps (seventh, augmented, or diminished intervals), choose small intervals. The leading note resolves to the tonic.
Leaps of a seventh are NOT allowed.
In major keys, diminished/augmented melodic intervals are NOT recommended.
Augmented 2nds and 4ths are NOT allowed.
6ths should be avoided.
Leading notes in dominant chords ALWAYS resolve onto the tonic of a tonic chord. (Bach didn't always do this, though!)
In a cadential 6-4, the 4 resolves to 3 and the 6 resolves to 5.
Always choose a semitone step if one is available.
5. The soprano line should have an interesting melody.
The best intervals to use are 2nds and 3rds.
4ths and 5ths are OK, but should only be used in an emergency.
Repeated notes can be used, but the more you use the more boring your melody will be, so only use them if you are stuck.
6. The alto and tenor lines should not move about much at all.
In the alto and tenor parts, you are padding out the chords: Always choose the nearest note that you can, without breaking any other rules (e.g. of consecutive or illegal intervals, etc.)
If possible, repeat the previous note. Otherwise, choose the next nearest note. Leaps of 4ths and 5ths are ok but use them sparingly.
7. Double the root or fifth in root position chords. Double any note in the first inversion chords. Double the fifth in second inversion chords. Double the third in diminished chords. Double the third with care in other chords.
8. Never leave out a figured note. Never leave out the root or third.
You can never leave out the fifth of a 6-4 chord.
Never leave out the third or the root.
9. Never overlap parts.
Don’t let your voice parts overlap. The soprano line must always be higher than all the rest, the alto must always be higher than the tenor. Be careful not to write parts that cross over like these two.
10. Stay in the accepted voice ranges and don’t put more than an octave between the upper voices.
Reference: https://www.cmburridge.com/teaching/bach-chorales/lesson-2-rules-of-chorale-harmony/#:~:text=Rule%201%3A%20Avoidance%20of%20Parallel%20Fifths%20and%20Octaves.,It%20becomes%20much%20harder%20to%20build%20...%20