So, why are the I, IV and V chords so important?

For this explanation, I'm hoping that you have a basic understanding of intervals and chords, if not, hold off until you've learned about intervals in the interval section.

Basically, all harmony in western culture is an artificial construct, based on hundreds of years of people figuring out what sounds good and turning that into habits. Back in the old days, and we're talking about the very old days, people were just coming out of the dark ages. Anything that even resembled knowledge seemed like something mystical and wonderful. Anyway, some of the earliest knowledge that started making its way back into the World came from Greece. Among the many philosophers and thinkers from Greece, one that had particular sway over many was Pythagoras.

Pythagoras was a mathematician responsible for discovering many things that are very common place in today's math. However, he also held that numbers were mystic to large degree. So, this whole school of mysticism sprung up around Pythagoras. Some of his mathematical theories applied to music and the mathematics of vibrating strings. For Pythagoras, the relationship 3/2 created the interval which we know as the fifth. For Pythagoras, this relationship had certain mystic qualities.

Anyway, let's move forward to around the year 1200 or so. The main places where knowledge was being sought and stored was monasteries. Monasteries are usually occupied by monks (a little joke). Some of these monks got a hold of the teachings of Pythagoras and thought that they were very cool, and this whole idea that music and mathematics could be joined and become something mystic. They were also in particular awe of this relationship 3/2, cause to them, the number 3 signified the trinity and other things spiritual. Remember, these are monks, and there was no pop music, if you had music, it was going to be sacred music. So, since this relationship of 3/2 yielded the interval of the fifth, they decided to build a whole tuning system around fifths.

This system of intonation built on fifths became known as Pythagorean tuning. In a system where fifths rule, fifths are the main consonance and also, fifths are the main interval. Movement from one chord to another is also in fifths, and those chords that are a fifth away from the main key center are going to be considered the strongest. If you click here you can see that if you're in the key of C, a fifth going up gives the note G and a fifth going down gives the note F. So most of the chords in your composition, if you're in the key of C are going to be the C, F and G chords. The 2 strongest chords are the C and the G.

Pythagorean tuning and tunings based on this system were around for hundreds of years and during that time the habit of moving chords in fifths was firmly established. There was another good reason for chord movement in fifths. Chords moving in fifths yielded the greatest number of common notes and nearest notes, so harmonic movement was very smooth.

There was only one drawback to Pythagorean-based intonation. It didn't work for all keys. If the intonation was built around the C note, the keys of Ab and Gb sounded awful. They were very out of tune. So, if you were a composer in those days you avoided those 2 keys. Another thing is thirds didn't sound that good either in Pythagorean intonation, so a lot of the masses written during that time, the last chord is usually just open fifths, with no third.

Anyway, at some point, people I'm sure got fed up with the fact that you couldn't write in all keys with Pythagorean intonation. In the 1500s, a method of intonation called meantone tuning came into being, which was more flexible than Pythagorean tuning, but not as pure. The Renaissance was kicking in around this time too and people were experimenting with all sorts of things, including new music. By the 1600s, meantone tuning gave way to a a new tuning called equal temperament.

Equal temperament basically made all the notes a little out of tune. But thirds now sounded better and all keys could pretty much be played, even though they were all a little out tune. Some composers at that time embraced this new equal temperament, particularly one by the name of Johann Sebastian Bach. He went ahead and wrote a couple of works for keyboard in all keys. Among these were the Well Tempered Clavier Books I and II and the 2 and 3 part inventions.

And in all of these works, as in the works of those that came after Bach, like Haydn, Beethoven, Mozart, to name a few, this relationship between the I and the V chord was still supreme,this was the one chord progression to top all chord progressions, as well as various progressions of chords in fifths.

And so, when you look at the music of western culture, those 3 chords still rule folk music, blues, rock and roll. It's a tradition and a culture that has been handed down to us through hundreds of years of use. And it still works fine today, if one is to look at the number of songs written just using 3 chords.