My father was a great musician, with a beautiful voice. He played several instruments from Cello to Trombone, and piano, guitar, and organ as well. He was the one who got me started on playing the piano and guitar by ear. He taught me thus basic fundamental truth about playing by ear.
Here goes: There are 3 basic chords that you can use to play any song, in any key. They are the chords formed by the First (1), Fourth (4), and Fifth (5) degrees of the scale. Using these chords may not be what the original composer may have envisioned for their masterpiece, but the three chords can be used to play any song, IN ANY KEY.
For example, in the key of C Major, which doesn’t utilize any of the sharps or flats, the scale runs through the following notes: C, D, E, F, G, A, B, and then back to C. The first note of the scales is C with the corresponding chord of C Major. The fourth is F, with the corresponding chord of F Major. The fifth note on the scale is G with the corresponding chord of G Major.
The notes C, E, and G, make up C Major
The notes F, A, and C, make up F Major
The notes G, B, and D, make up G Major
Now that you know this, the important thing to remember is that each chord has 3 notes in it. The trick is to play the chords that correspond to the notes in the chord. This is usually a good idea. However there exceptions to this rule. Sometimes holding your chord is the right thing to do instead of changing on each note that is different from the chord.
That being said, it’s time to smile and continue to Play It By Ear!