Jazz - drop 3 and drop 2 voicings
The 3-strings voicings (a.k.a. shell voicings) are very common in jazz and perhaps even more than the regular full version shapes, especially in comping. The advantage is mainly that they are easier to play and memorize since they only include three notes. They are often a great choice for comping, for example in Freddie Green style comping (meaning rhythmically four downstrokes for each bar in staccato quarter-note rhythm with swing feel).
What to observe is that they all emerge from 4-note chords. What happens from a theoretic perspective is that one note, the fifth which is the least important for the sound character, is omitted. This results in a chord including three instead of four notes – the “shell”, or the skeleton of the chord, so to say.
The 5th string is often muted and generally by the finger on the 6th string. Also, the strings surrounding the strings that shouldn't be played are muted by an adjacent finger.
Drop 3 voicings
One of the main concepts of chords in jazz is drop 3 voicings. Drop 3 voicings refer to chords in which the third note from the top is dropped down an octave. This also change the chord from a closed voicing to an open voicing. To better explain this, Cmaj7 can be used as an example. The closed voicing version of Cmaj7 includes the notes C, E, G and B and they could be stacked as B, C, E, G. To change this into an open drop 3 voicing, the C note would be dropped one octave, as the figure below illustrate.
Examples of drop 3 voicings:
maj7 shape
m7 shape 1
7th shape
Drop 2 voicings
The same concept is involved in drop 2 voicings. Drop 2 voicings refer to a chord in which the second note from the top is dropped down an octave. The closed voicing version of Cm7 includes the notes C, E, G and B and they could be stacked as G, B, C, Eb. To change this into an open drop 2 voicing, the C note would be dropped one octave.
Examples of drop 2 voicings:
maj7 (shape 2)
If you want to go more into depth of this music style, see The Jazz Guitar Chords ebook with over 250 chord diagrams.
See also Jazz chord progressions.