I would not be so quick to assert that a song's unaccompanied melody (its "air"?) is always its *foundation.* Depending upon the tradition from which it descends, or upon the individual piece in question, the instrumental part might very well be the more fundamental element of a given piece. Consider, for example, just about any blues number -- e.g., Robert Johnson's "Dust My Broom." And even in cases where no particular intrumental "riff" is truly definitive, there are many cases where a song's basic harmonic structure (the chord progression) is far more elemental, more truly the song's "foundation," than its melody.

There's also the question of the learner/performer's intention; when reinterpreting a *particular version* of a traditional song already well-known in several different guises, one might well start by learning the instrumental part and only later working in the vocal part.

It has been pointed out that different sets of chords can be used for the same melody. Does this mean that a song's harmonic structure is somehow "accidental" whereas its melody is essential? I would say, ceratinly not! A song, in many contexts anyway, is defined both by its melody and its harmony. A single melody played with two different sets of chords can become two different songs -- fundamentally different. (We can assume there'll be different lyrics, too, of course.)

A while back, we had concurrent discussions going on here about Mississippi John Hurt's "Let the Mermaids Flirt with Me" and Singing Brakeman Jimmie Rodgers' "Waiting For a Train," two songs written at about the same time, in the same state, following the same melody. The melody was not brand-new to either artist by any means, but came from some long-ago presumably "traditional" source. One of the songwriters in question is generally acknowledged to be a "folk" artist, while the other is recognized as the "Father of Country Music," and therefore generally classified in a gray area between the "folk" and "commercial country" categories.

"Mermaids" uses a basic three-chord arrangement featuring a fingerpicked melody line. (Pedants can feel free to assign a different chord name for every note added to a chord, but most of us would consider it a 3-chord piece.) "Train," on the other hand, uses a far more complicated chord structure, akin to a trad-jazz tenor banjo part; in fact, it's so complex that the easiest key in which to play it, believe it or not, is F!

I have learned both the above-referenced songs, and in both cases worked out the intrumental part first, or at least at the same time as I practiced the singing. There's no way I would consider the unaccompanied sung melody to be the "foundation" of either one; in fact, on the evidence of the melody alone, they are not two distinct songs at all, just two interchangable sets of verses sung to the same tune!

Of course, the point of view opposite mine, the one I'm arguing against, is undoubtedly correct -- BUT, in another context!