In this article https://www.guitar-chords.org.uk/c-major9-chord.html
Why is the 5th missing in the first and second shape, and the 3rd missing in the third shape?
Why are sometimes all the deegrees included and sometimes not?
Thanks
Most of the time it’s because of fingering constraints, we just don’t have enough fingers to reach all of the notes so sometimes certain notes are left out. Where possible it would be the most neutral notes, like the 5th degree that gets omitted, but not always.
Some styles of music, jazz particularly, guitarists will use chords consisting of only three or four notes even where the fingering is possible. This gives a different kind of sound and voicing possibilities without necessarily losing the tonal quality of the chord.
In a band context where there are other instruments playing, it’s less important to play all of the notes, for instance if the bass player is playing the root note then the guitarist could leave out the root of the chord, the note will just be coming from a different instrument. The more instrumentalists there are in a band the more reason there would be to play fewer notes so that some tonal space is left between instruments and things can sound cleaner overall.
Generally speaking, if you omit notes from a chord, as long as the important notes are given priority then we still get the sense of the right sound. For example if we include the root and third then we can determine whether the chord is major or minor, if we then add the most important note, i.e., the 9th if it is a 9th chord, then even without the fifth it should still sound like a 9th chord most of the time.
