When you play rhythm guitar you are doing what we say is comping. It means you are in a support role and playing the chords behind the singer or lead instrument. Keyboard players comp, too.
So what about the voicings you play when you comp? Well that depends on the genre and it’s a very important to play what’s expected for the genre.
If you’re playing in a situation where there are lots of 7th chords, and there’s a bass player, it’s good to learn shell voicings. Shell voicings have only the most defining chord tones of a 7th chord, and those are the root, 3rd and 7th. The root is the fundamental note of the chord, the 3rd determines the quality, and the 7th defines the quality in more detail.
And if you’re playing with a bass player, they’ve got the root and you can cover the 3rd and 7th which are the other notes of the shell voicing. This may seem crazy, but playing just 3rds and 7ths can have a really full sound. And adding an extension like a 9th or 13th to a shell voicing is an even fuller sound.
If you want to know the ‘whys’ behind all this, check out Fretboardbiology.com
