Reading through this thread, I get the sense that not everybody is talking about the same thing.

I've heard people refer to an ordinary Dominant 7th chord as a "major 7th" because the basic triad is major. Not accurate. A Dominant 7th is a major triad with a note added. The added note is an interval of a minor 7th above the root [Example—G, B, D, plus F]. A major 7th chord would be a major triad with an added note a major 7th above the root [Example—G, B, D, plus F#]. For all practical purposes, there is no such thing as an interval of a diminished or augmented 7th. A diminished 7th would be the same as a major 6th and an augmented 7th would be the same as an octave.

Classical musicians and musicologists consider that the triad B, D, and F does constitute a diminished chord. A triad is a basic chord stripped to its bare bones. Jazz musicians and people who learned what music theory they know out of a guitar manual usually assume that a diminished chord has to have the 7th added because that's what the chord diagrams usually show, and that's the way they usually use it. A diminished 7th chord a sort of "catch-all" chord. Since any one of the four notes can be taken as the leading tone, it can resolve to any of four different chords, making for easy modulations. A dominant 7th (which contains a diminished triad) resolves to only one chord (the tonic). To further add to the confusion, the 7th in a diminished 7th chord is the exception to the idea that there is no such interval as a diminished 7th. The added note is a diminished 7th, (enharmonic with a major 6th), but if you don't "spell" it that way, it could lead to endless confusion in the way subsequent chords would need to be written—assuming you're writing a fairly formal composition. This is sort of the "basic spelling" part of music theory. Knowing all this would be beyond the practical needs of most folk musicians. But not knowing it can lead to confusion when trying to communicate with other musicians.

I do use major 7th chords from time to time, but sparingly and judiciously. They're very lush. In the version of The Braes of Yarrow that I do, I use an F major 7th (F, A, C, E) just before the final Am in each verse. It would give a purist wall-eyed fits if he knew what the chord was, but boy, does it work!

The ultimate test is how does it sound?

Don Firth