Music theory allows it to be so!
I have met students who have played guitar for years and have not been able to play a single song! Wow... Why does that happen? I have also seen a guitar player sit and play song after song and never use more than 4 chords. I once saw a guy who had won awards for songwriter of the year do a short 5 song concert and never used more than G, C, D and Eminor. He used a capo to change keys and just kept strumming the same chords.... song after song!
So, if you have been struggling to learn an instrument like guitar or piano and you really desire to play your favorite songs, it really can happen quickly if you just learn these four chords!
G, C, D and Eminor
There are many sites and books to show you the chord forms. I am going to explain the music theory that allows this to work.
First, in the major scale there are only 7 notes. We will us C major because it is easy and has no sharps and flats.
Starting on C and counting up on the white keys of the piano it looks like this:
C D E F G A B C
Once you build a chord on top of thes simple notes of the major scale you have these chords
C Dminor Eminor F G Aminor and Bdiminished!
Now, if we take a simple song with a chord progression like this C F Am G
we could asign a number to each chord in order from C - C
C=1 Dminor=2 Eminor=3 F=4 G=5 Aminor=6 Bdim=7
Now our chord progresson C F Am G can be written in numbers like this
1 4 6 5
Once you begin to look at songs like this in numbers and patterns, you can play any song with this simple chord progression
Just change the key to G and you have this formula:
G=1 Aminor=2 Bminor=3 C=4 D=5 Eminor=6 F#dim=7
Now, you can play that 1 4 6 5 chord progression using G C Em D
You can "transpose" every song in the world to the simple to play key of G on the guitar and play and sing every song with just about 4 chords. That is because most pop, rock and country songs only use the 1 4 6 5 chords
If they happen to get crazy and use a 2minor or 3minor chord, no problem, you can do it!
You may be thinking I am crazy but hundereds of Beatles songs, Elvis songs, and about every song you hear on the radio follows this formula in differnt orders but using the same simple chords!
Do a youtube search for 4 chord songs and you will be amazed! The band Axis of Awesome just put out a great video with 4 chord songs and sang forever through tons of songs with the same simple chord progression!
So, get those 4 chords out and start rocking and strumming along to many great songs!