I wonder if Jimmy is talking about accompanying Irish or Scottish or old-time tunes, where the mode may be other than a standard major or minor. Some guitarists (better than I) will play a chord without the 3rd, giving a somewhat indeterminate feel to the chord and fitting in better with the tune. I once heard such a chord, in A, for example, referred to as "A-neutral". (It might have been Jere Canote of the Small Wonder String Band.) I'm sure Rick Fielding could give a much better explanation than this, including how to play the thing.