We hear this question from new guitarists all the time.
Aside from the theory, which we’ll get to in a moment, the main difference between major and minor chords is how they sound. Major chords have a brighter, happier sound while minor chords produce sounds that are sad and melancholy. Songwriters choose their chords carefully to evoke emotions in the listener and give musical form to their lyrics. As you become a more experienced player, you will likely be able to tell exactly what chords are being played within a song.
Now, about the theory: The difference between major and minor chords is this: A major chord consists of a 1st, 3rd, and 5th degree of a major scale while a minor chord is made from the 1st, flatted 3rd, and 5th degrees of a major scale. So the only difference is the 3rd degree. You can see this clearly when you compare certain chords such as an open A Major with an open A Minor chord. The only difference is the note on the second string; changing that one note profoundly affects the sound.
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