Major and Minor chords are built with three different notes, where as 7th chords are built with four different notes. For better understanding of the 7th chords, you better watch the video at the bottom of the page.
Major Chords
Note: The major chord is built with only three different notes. For example, let us take C major. The C major scales are C, D, E, F, G, A, B. The formula of major chord is to take out the 1st, 3rd and 5th notes from its scales. So in case of C major, the 1st, 3rd and 5th notes are C, E and G respectively. If we play these three notes at a time, a major chord will be formed. Every different major chords has different scales. So for every major chord, the 1st, 3rd and 5th notes are different and hence they sound different.
Minor Chords
Note: The minor chord is also built with only three different notes. For example, let us take C minor. The C minor scales are C, D, D#, F, G, G#, A#. The formula of major chord is to take out the 1st, 3rd and 5th notes from its scales. So in case of C minor, the 1st, 3rd and 5th notes are C, D# and G respectively. If we play these three notes at a time, a minor chord will be formed. Similarly, every different minor chords has different scales. So for every minor chord, the 1st, 3rd and 5th notes are different and hence they sound different.
Maj7 Chords
Note: A Maj7 chord is simply denoted by the note name followed by maj7 (eg: Amaj7). Maj7 chord is simply a major chord with additional 7th note. The major chord is built with 1st, 3rd and 5th note. Hence adding the 7th note to major chord creates Maj7 chord. For example, The C major scales is : C, D, E, F, G, A, B. Now lets take out the 1st, 3rd, 5th and 7th note i.e. C, E, G and B.
Dominant 7th Chords
Note: A Dominant 7th chord is simply denoted by the note name followed by 7 (eg: A7). A Dominant 7th chord is very similar to maj7 chord but only one different note. If the 7th note of major chord is flatted, then the dominant 7th chord is created. For example, the 1st, 3rd, 5th and 7th notes of C major scale are C, E, G and B. Now flat the 7th note i.e. B note, it becomes A#. Lets play the C, E, G and A# together, the C7 chord is formed.
Minor 7th Chords
Note: Minor 7th chord is simply denoted by a note followed by m7 (eg: Am7). Minor 7th chord is simply a minor chord with additional 7th note. The minor chord is built with 1st, 3rd and 5th note. Hence adding the 7th note to minor chord creates m7 chord. For example, The C minor scales are C, D, D#, F, G, G#, A#. Now lets take out the 1st, 3rd, 5th and 7th note i.e. C, D#, G and A#.
