The “80/20 Rule” states that 80% of results or rewards will come from 20% of causes or effort. Put another way, 20% of input creates 80% of output. This especially applies to music, where the same chords and progressions repeat themselves over and over.
Think about it — how many times have you played the same chords or patterns across different songs? Granted, if you don’t understand your scales and number system, you won’t know you’re playing the same chords and patterns because they’ll be disguised in another key.
For example, a C major chord progression using the chords “C major – F major – G major” is the same thing, relatively, as an Eb major chord progression using “Eb major – Ab major – Bb major.” Knowing the number system allows me to put these two seemingly different chord progression in a single, universal language.
They are both 1-4-5 chord progression in their respective keys. C major is the 1st degree of C, F major is the 4th degree of C, and G major is the 5th degree of C. Likewise, Eb major is the 1st degree of Eb, Ab major is the 4th degree of Eb, and Bb major is the 5th degree of Eb.
I mention the “1-4-5” chord progression because it’s one of those few 20% patterns that make up 80% of music. (Surely, the numbers aren’t exactly “80%” and “20%” but the idea is – to put it in another way – a very small percentage of ingredients are used in a wide variety of recipes.)
It would be a very good idea to take inventory of all the chords and progressions you play in songs, reduce them down to their numbers (e.g. – “1-4-5, 2-5-1”), and see how many repeat themselves across different tunes and even genres. You’ll definitely see the 80/20 rule at work.