The basic chords of music are TRIADS - 3-note chords composed of various intervals of the scale based on the root, 3rd, and 5th and their alterations. To form a 4-note chord you simply add a note of the scale such as the 6th, 7th, or major 7th to the basic triad. This short video demonstrates how this works:

For a COMPLETE course in chords click on “All The Chords In The Whole Wide World!”

Piano chords come in all flavors — major, minor, diminished, augmented, and many kinds of extended chords such as 6ths, 7ths, 9ths, 11ths, 13ths, etc. This 50-minute video will give you a good introduction to piano chords and prepare you for more in-depth study:

Chords of all kinds: Major, minor, diminished, augmented & extended chords from chordman on Vimeo.

6th chords are 4-note chords consisting of the root, 3rd, 5th, and 6th notes of a major scale. So just by adding the 6th degree of the scale to any major chord results in a major 6th chord. Watch this short video:

For more information, please go to http://www.playpiano.com/101-tips/8-major-6-chords.htm