I came from reading a short piece found in internet, "Simplifying the fretboard with Pat Martino" by Jude Gold and found there a very nice insight from Martino. Learning a guitar chord vocabulary can be overwhelming, but his idea is that you can stack minor thirds to form a dim7 chord. Say you get the pattern to play it in the middle 4 strings. Then you flatten each one of the strings. Thus you generate 4 patterns of a dominant seventh chord! In fact you generate all the four inversions of the dominant seventh. Additionally those patterns match in 4 of the 5 CAGED forms. And that are the drop-2 patterns, and the relation, I think, is valid for all other drops. Having mastered dominant sevenths, one derives maj7, m7, and m7b5, the other diatonic chords, by altering the dom7, and all or most are realistically/fingerly playable. And also a similar trick for triads. All moveable of course. Thanks, Pat Martino!
> Mulhern had taken guitar lessons from Martino and often, to Martino’s aggravation, had played a major seventh chord when Martino wanted a minor ninth.
This is an instance of the Picardy Third, a trick that sounds really hilarious when you insert it into a piece where it doesn't belong.