Originally posted by griphon2
A C lydian dominant is a G melodic minor (from C if you want to look at it that way). Or a G maj scale with a b3rd from the 5th of C7. Always. Lydian is always IV.
Or the major scale from the 5th of the chord.

Yeah, lydian is the IV when your thinking with the major scale primiarly in mind. My point is the lydian scale itself is a completely different scale and it has no relationship with the major scale what so ever. I know how much you hate Roots of scales, but you have to look at like that. Tonality is the key, a lydian scale sounds different from a major scale. Same with the lydian dominant and the melodic minor. They may have the same notes, and I know you know this. If you play something in C major, then play it in A minor. It sounds different. All scales act differently in the same way. So if your saying lydian is always IV, your actually playing a major scale(or at least thinkin in terms of it).

The lydian dominant is the same way, if it's a IV it's the subdominant of the melodic minor. When actually the lydian dominant scale is called that for three reasons; one is the root can be written as a dominant chord.

Take C lydian dominant.
C D E F# G A Bb
C E G Bb = dominant chord

The second reason, is because of the #4 which is a recognizable part of the lydian mode. The flat 7th is there from the dominant purpose(root).

There is also another dominant chord in there. D F# A C.

And last, the reason you said. The lydian mode is always a IV in a major scale. But the scale itself (lydian dominant)doesn't act as a IV, it has it's own set of chordal structure arrangement. ( I II iiio ivo v vi VII+ )

Here's the melodic minor: i ii III+ IV V vio viio

I know alot of theory teachers and books have this all screwed up. Simply because modern ideas on modes is that they only exist within there diatonic scales. Now, culturally we are a prodominantly major/minor oriented; the true use of modes is hardly un-opened.

The way you find scales on the fretboard is absolutely correct (still think it's alittle harder, but hey... :) ). I did something like that when I started, if I needed to find a melodic minor scale. I would just play a minor scale and raise the 6th and the 7th. Or easier, play a major scale and just flat the 3rd. Now that I've been playing for sometime, I just know them uniquely by there pattern. You shouldn't have to think of a different scale to find another. Too confusing, when it's so simple.