The Big Deal With Parallel Fifths (and Octaves)

After recently having dived deeper into why not to double thirds in a chord too extensively, this time I'd like to shed some light at the infamous fifth and octave parallel avoidance rule.

Unfortunately this is another of these rules that we just learn to accept without asking why. We do this to an extent that some music theory teachers can not explain the reasons behind this rule.

The rule originates in a time where polyphonic music was very popular and had its peak in the baroque era. All the basic rules for four part writing originates from this time. In this time it was desirable to have all voices have their unique melodic quality, the horizontal independence of the voices was prioritized really highly so it was desirable to be able to follow and perceive each voice independently.

On the other hand do the intervals of a fifth and an octave between two notes that sound at the same time have a high degree of consonance which also means that they tend to melt together in our perception. In fact there are quite a few studies which show that if you play two notes an octave apart to musically uneducated people the majority will identify this as one note and even with perfect fifths this percentage is quite considerable.

Putting these two factors together exlains why it would be counter productive to have voices that you want to be perceivable separately to be moved in fifths or octaves as the degree of consonance with these intervals would be so high that it would compromise the ability to distinguish these two.

Having said that, it also becomes clear that this rule only applies in very few cases in current music. In music where you clearly have a separation between melody and accompaniment it is clear that the accompaniment doesn't need to provide and individual line that an be perceived as its own melody. In fact some arrangement techniques (for instance in Big Band block writing) use this parallelism as stylistic element and create a specific musical colour by doing so.

Also in orchestral writing the need to create an individual melodic quality for every instrumental line is usually not desirable. In fact our brain struggles with following more than 3 to 4 individual musical elements.

Still there is some use that we can get out of this rule. Firstly, it sharpens your musical understanding and control practicing 4 part harmony properly, so even if you don't use it in your day to day work, it is beneficial to have enough control over music to be able to use it properly. 

Also, it is not uncommon to work with counterpuntal elements, be it as a side line for a theme or a stage of development where such rules do come in handy. Even if the strict following of these rules is not necessary, there should be a simple formula that you should follow in general with any rule in music: It is perfectly fine to ignore them deliberately, but not by accident. 

And lastly, melodies that are lead a perfect fifth apart for a long distance really do sound quite pedestrian.

So the bottom line here is: Nobody needs to be ashamed for fifth or octave parallels in current music but once in a while making use of these rules can be beneficial.