Major chords are formed from major scales by taking the root, 3rd, and 5th of a major scale. But to know which notes are the root, 3rd, and 5th, you need to know how a major scale is formed out of whole steps and half steps.

Here is the formula:

Start on any note, then go up a whole step, whole step, half step, whole step, whole step, whole step, half step until you get to the note an octave above where you started.
It’s easy to see in the Key of C, since all the piano keys are white keys. But in any other major key, there will be one or more black keys in the scale.

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