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After the unison and the octave, the perfect fifth is the most consonant interval. It has a frequency ratio of 3:2, which means it beats very nicely to our ears. It’s classified as a perfect consonance.
It is seven half steps, or seven frets up on a guitar.
The pure perfect fifth can be played with the harmonic on the seventh fret. The equal tempered perfect fifth, which is what you get when you play the seventh fret, is just slightly flat of a just perfect fifth. The difference is slightly less than 2 cents, which means that all but people with the best pitch sense will be unable to hear the difference.
The Theme from Star Wars and My Favorite Things both start with ascending perfect fifths. The Flintstones Theme and Bach’s Minuet in G (the one every kid learns first on piano, or How Gentle is the Rain), both start with descending perfect fifths.
The perfect fifth is the rock or blues bass players friends. If you can play the root and the fifth of each chord as they come, you are good to go for most jamming situations as a bass player (since all the rest of the world wants to be guitar heroes). Think of the opening bass notes to, for example, Rikki Don’t Lose that Number – its nothing but rocking back and forth from root to fifth.
I have attached a pdf of perfect fifths in open and closed positions. There are only four shapes for perfect fifths, so they should not be too difficult to learn. The cool thing to notice is the first five open perfect fifths. In the bass, they have E, G, A, C, and D. Re-arrange, and you have CAGED, which is the system so many use for learning their basic chords, and also the way Brian arranges his pentatonic scale positions. This is not an accident, and has to do with how these fifths naturally occur for an instrument tuned to perfect fourths. But it is cool. Also, all sorts of songs can be made up by playing with the order of these five fifths (Hey Joe immediately comes to mind).
Early music, meaning Gregorian Chants, first confined themselves to singing mostly in unison or octaves. Gradually, a new style called organum developed. It would allow for other perfect harmonies to intrude, and even for a melody line to move over a stationary pedal point. This style relied heavily on the fifth. Here is an early example from the great composer Leonin, dating back to about 1175.
I’ve also attached a pdf of a major scale of harmonized fifths. All but one of the fifths in the harmonized scale is a perfect fifth, and this makes it easy to run these in parallel by just moving your hand shape up and down the fretboard.
As harmony developed, the fifth would be less important. One of the key principles of the movement of voices is that you should not have parallel fifths, which means two consecutive chords that have fifths that are parallel to each other. (This is a sin that we commit all the time on the guitar, but its not really as big a deal as music theoreticians make it out to be.) The trouble with parallel fifths is that they become too strongly interdependent on each other, so it becomes to break out of them once you get into them. (A music theory teacher would have nightmares if you played a lead using the harmonized scale I have attached here.)
Eventually, with some jazz, the fifth becomes one of the least important intervals. When jazz musicians accompany people on the guitar they focus first on the 3rd and the 7th, and then on upper extensions of the chord. They might play the root, especially if there is no bass player. But, when deciding what notes to play, the fifth is almost always the first note of a chord that the guitar player gets rid of. An example is when we play a C7 chord in first position. That voicing, which is very common, leaves out the fifth.
So, the fifth went from being one of the most important intervals in early music, to being one of the most neglected. That is, at least until heavy metal resurrected it, by introducing the power chord. A power chord is basically the root, fifth and octave, and leaves out the third. It basically takes us back to the days of early music and the gregorian chant, but with distortion and head banging.
Thus, we can see how civilization has progressed.
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