More Chord Theory

OK. You've seen how to form all the major chords from the root, 3rd and 5th
but you will have noticed that the mandolin family has got four courses of strings.
What do you play on the fourth string?

Well you *should* start with the root note on the lowest (G) string, so that means that its the highest (E)
string that gets left out.
Two things though. First, playing the root note on the lowest string isn't always convenient and second,
duplicating one of the three notes (root, 3rd or 5th) can make a pleasing addition to the chord.
If you really want to impress people you can call this 'triad doubling' but really what you want to know is this:

If you double the root, it reinforces the chord, stressing the root note itself
If you double the 5th, it increases the stability of the chord
If you double the 3rd it emphasises the tonality (major-ness or minor-ness) of the chord

The doubled note can be an octave above if that's the most convenient to play.

See Inversions for more on playing root notes other than on the lowest string
and see List of Major Chords for a complete set of full major chords.

Ready for some more theory? How about how Minor chords are formed?