I think of 6/8 (instead of 3/4) when the feel of the measure is 6 full beats that can't properly be subdivided in two 3/4 measures - like a blues song in 6/8 where the backbeat is on the 4th of 6. (e.g. "Steamroller Blues" or "Life by the Drop"). My guess is that it would be a similar thing for classical music. (minus the backbeat)
6/8 is mostly beat in two, 3/4 in three. While 6/8 does consist of two groups of three eighth notes, they're more like triplets in 2/4 than two measures of 3/4, so you'd beat it like it was 2/4. But 6/8 is just easier to write/read than triplets, especially when using dotted rhythms: (ex. dotted eighth - sixteenth - eighth)
I could be totally wrong so correct me if so... and don't know if this will help so just throwin it out there.
I have been playing with a keyboard a lot lately, because GPO has Steinway in it, and I just discovered a common chord progression that i believe would require 6/8 because it is Arpeggiated (thanks Gary ) with 6 notes.
The song "Unchained Melody", if played in C, has the chords:
C, A & F descending, and then ascending to G and back to C.
(edit: changed the way i wrote that to make it clearer)
with each chord apeggiated with root, ascending to third, fifth, and root's next octave, then descending back to fifth and then back to third, making six notes so I'm pretty sure it would be in 6/8.
I was also pleasantly suprised to see that if you speed it up or slow it down and tinkle around with some righthanded high notes, you find a lot of those old 50s and 60s tunes are on that chord progression.
You're talking about a figure such as
ceg c'ge Ace aec FAc fca GBd gDB
(where the spaces represent groupings)?
If so, then yes, you could indeed notate in 6/8. You could also use 12/8, or triplets in a meter such as 2/4 or 4/4.
Not sure if you are talking to me, but I edited it to make it clearer.
In iTunes about the tenth one down is a John Williams arrangement of that song, in the sample I didn't hear the chords, but it would be interesting to find his score and see what he used for it. He maybe used one of those you mention.
I was talking to you, and unfortunately your edit did nothing to make it clearer: I was asking about the rhythm of the figure, not the particular chord progression. I gather that I understood correctly, though.
OK, i see now what you meant, i'm a noob and was unfamilar with those rhythm representations you posted. It's played softly and evenly, and no note has stronger accent than the others as near as i can tell.