As a result of playing guitar at a very young age, I learned a lot of theory related to playing by ear, guitar lends itself well for that. It’s fairly easy to learn a lot of chords and put them together in progressions. I’ve met a lot of piano players that started reading music at a very young age, and they can sight read almost anything, but take the music away from them and they can’t play. Apparently, they were never taught to play by ear. I’m posting this in order to help this type of piano player to learn by ear. I taught myself to play piano by ear using the following chord theory. I started playing piano as an adult and I found it easier and faster to play by ear. Everything on a piano is linear and chords can be constructed from the Major Scales. Chords that go together well are the Tonic, Sub-Dominant, and Dominant chords, or the 1st., 4th., and 5th. chords of the scale. Many blue songs, rock songs, and popular songs use this chord progression.
Major Chords are constructed using the 1st., the 3rd., and the 5th. notes of the scale. For example, in the key of “C”, the notes in a C Major Chord are the C, the E, and the G, or the 1st. note of a C Major Scale, the 3rd. note of a C Major Scale, and the 5th. note of a C Major Scale. This is called the Tonic chord, often called the root chord, or first chord.
The notes in a F Major Chord are the F, the A, and the C, or the 1st. note of an F Major Scale, the 3rd. note of an F Major Scale, and the 5th. note of an F Major Scale. This is called the sub-dominant Chord, often called the fourth chord, the “F” being the 4th. note of the C Major Scale.
The notes in a “G” chord are the G, the B, and the D, or the 1st. note of a G Major Scale, the 3rd. note of a G Major Scale, and the 5th. note of a G Major Scale. This is called the dominant Chord, often called the fifth chord, the “G” being the 5th. note of the C Major Scale.
Minor Chords are constructed using the 1st., the flatted 3rd., and the 5th. note of the scale. As an example, in the key of C this would consist of the C, the E flat, and G notes. This would be called the C Minor Chord.
As follows the “F” Minor Chord would contain the 1st. note of the F Scale, the flatted 3rd. note of the F scale, and the 5th. note of the F scale or the F, the A flat, and the C.
The “G” Minor Chord would be the G note, or 1st. note of the “G” Scale, the B flat, or flatted 3rd. note of the Scale, and the D, or 5th. note of the Scale.
Major and Minor chords can be altered into 6th chords simply by adding the 6th tone of the scale. Major 7th chords would be played with an added 7th tone of the Major scale. To make a chord into a Dominant 7th you have to flat the 7th tone of the scale, however, this is almost never called a Dominant 7th., the Dominant is dropped, and, as an example, the chord would be called a “C7”, or “F7”, or “G7”, or “Cmin7”, “Fmin7”, and “Gmin7”. Diminished chords are flatted thirds stacked on top of one another or a C note, a E flat note, a G flat note, and an A note. Diminished chords are the only chord that can be named from every note in the chord, therefore every note is a root note. As an example the previously described diminished chord can be called a “C dim.”, a “E flat Dim.”, a “G flat dim.”, or a “A dim.” An Augmented chord has a sharped 5th note of the scale so a “C” chord would consist of a C note, a E note, and a G sharp note.
In order to play proficiently by ear all chords must be memorized in all keys. I would suggest to start either chromatically or practice the chords in the circle of fifths. Also learning to voice the notes in different order is an absolute must! By voicing the chord properly you can use a fake book and keep the melody in the higher register and fill in the harmony with chords. The left hand can be used mostly to play octaves, fifths, and occasional bass runs or arpeggios. Until you become proficient in all keys you can focus on the key of C which is the easiest key to learn because the “C” Major Scale has no sharps or flats. Speaking of scales, you must learn all Major scales backwards and forwards with both hands. It’s also advantageous to learn as many scales as possible including, but not limited to, minor scales, pentatonic scales, and blues scales.