Drop 2 voicing explained!

Drop 2 voicing is one of the most popular voicing used in the guitar.  Its harmonic structure lends itself very nicely for the instrument.  An example in "A major 7" is outlined below:

Standard voicing from the bottom of the music stave: A C# E G#

Drop 2 voicing:  you drop the second voice from the top.  The top is G#, so the second note (the one following that G#) is E.

Therefore, now you can play E A C# G...the 5th, Root, 3rd, and 7th.

That's why when we play a chord in drop 2 voicing, we must understand that this chord is coming from standard voicing.  The issue with the guitar is this:  because of the way we tune, we need to tweak the chord voicing in order to make all the necessary notes.

Steps:
Arrange any chord in any inversion the classically trained way (in order!!!).

Take this standard voicing and drop the 2nd note from the highest sounding (drop the 2nd from the top).

Concluding thought:
That is why we can conclude that a major 7th chord that is voiced Root, 5th, 7th, and 3rd comes from the standard voicing of 5th, 7th, Root, and 3rd.  When you take this second inversion chord in standard voicing and drop the 2nd from the top (the Root), the you end up with the voicing we all make when playing a major 7th chord.