Having said all that, it's also worth remembering that "rules" in music reflect what has been found to sound right and work well most of the time, and one can find very good counterexamples to any of them.
But seriously, parallel motion at any interval -- but particularly at the octave or the fifth or fourth -- is not a really good idea.
Organum used parallel fifths and fourths,
Isn't Organum monophonic?
there are many caveats described in the excerpt from Michael's link… Passing tones, on the off beat; pauses between sections… or simply written down incorrectly … 18 'incidents' are cited - out of some 371 chorales --- This is hardly an endorsement by Bach for the use of parallel 5ths..
Hi Themeandvariation,The use of parallel fifths doesn't need J.S. Bach's endorsement. Mvh,Michael
Yes, originally my suggestion
Michael ...I never said they sound terrible…. But, one must use them Deliberately …as one chooses every note… The reason for being aware of such a rule, is akin to encouraging budding composers Not to always write their chords in the root position…. (of course one could if they want to, and they may say that it sounds good that way…) It is being aware of one's options…. and being able to hear the difference….
Yes. Finally, in the end there are no rules…or that the piece makes up it's own 'guidelines'… I believe the purpose of the exercise is to (more) develop the ability to hear each line as a distinct voice . When voices are in parallel motion, the effect to the ear is quite like 2 voices blending into 1 … which is fine, of course… Though if one is studying polyphony, the challenge (as an exercise) is to hear each voice as distinct - in order to develop that ability, should one want it..