The name of this chord can be confusing. In fact the structure of it is simple - a triad plus a dimished ninth. So it would not be totally wrong to name it Eb9 or E-9 but in first case it would seem that the flat goes for the...

B7 is a seventh chord and it is very often used because it is the dominant seventh chord of E-major which is one of the most often used keys on the guitar.

E9 is a ninth chord and a ninth chord is a chord with a ninth, and is an extended chord. Here the fingering is given so that you can use it to play all other ninth chords (X9). Just pick a right position.

Am9 is a minor ninth chord and belongs to the category of extended chords because it has more thirds than the seventh chord. What makes this chord a bit unusual here is that although it is played from the 5th position, it...