Parallel intervals have been around for centuries. They have a rich and detailed history. But what exactly are parallel intervals? Parallel intervals are any intervals (2 notes played together) that go consecutively in the same direction. For example, if you have the interval of a 5th – such as C to G; if you go to D and A that would be parallel intervals – in this case, parallel 5ths.
If you go back to the first written music (plainsong, also known as Gregorian chant) you simply had the singing of liturgical text – just one note at a time. When music eventually evolved to add a second note with the birth of organum there were many parallel intervals (specifically parallel 4ths).
As music developed polyphonically and counterpoint emerged (often with many simultaneous musical lines) the writings of Bach chorales and the basic rules of four part harmony actually forbade the use of parallel fifths and parallel octaves. But why?
The very first writing in music was vocal writing. Typically in four part chorale writing you have a soprano, alto, tenor, and bass and the idea was to have distinct separate vocal lines you could hear. As these lines intersected, harmonies were created. The reason that parallel octaves were avoided in this type of music is because if you have two different voices singing the same notes an octave apart it sounds like the same line.
This same principle applies to parallel fifths since they are sonically related. The same is not true for parallel thirds and other intervals. If you listen to parallel thirds and fourths you can still hear each individual part even though they blend well together.
I hope this is helpful and if you have any questions about this topic or any other, please email me Robert@LivingPianos.com for more information.