1.16An extremely important ear experiment

Play only two notes:

C + E

Then:

C + E♭

Don't add G.

You can already hear major versus minor.

That's because the third contains most of the identity.

Later, when we play jazz chords, pianists frequently omit the fifth entirely.

For example, a left hand might play only:

C + E + B♭

and your ear understands C7 just fine.

So start paying special attention to thirds.