With guitar finger forms, root shapes and individual chords we can derive another chord form called “bar” chords.  Basically a bar chord is used in place of the standard form when location and speed are issues in combination with ease of root note location.    

            Frequently used “Bar” chords are E, A and D major.  These are the most common forms because the three can completely cover the 6^th, 5^th and 4^th string roots as well as leaving the fingers in place to complete 60+% of all Lydian and Pentatonic fingering forms with ease and grace.

          As noted over the E-Bar chord, the root note on fret 5 is A, if we look at the A Major music staff and compare this to the E-Bar chord fret 5 staff we notice the similarities.  The only real important difference being established here is the finger / fret board positions relationship.