Nanohedron wrote:if the tune's evident chord structure is simple enough you can nail it fairly easily.
If it has one. And some tunes do. Others don't or, as suggested back in my second reply above, invite alternative (not necessarily mutually compatible) readings depending on other factors. And, granted that I'm departing from your point here, some might defy just about all attempts to 'fit' chords at all...
See if I'm writing tunes, I generally conceive tune and chords together as an entity from the word go, as many trad tunes may subconsciously already do in terms of implied harmony through outlining triads, sequences, expected cadences etc. in the melodic line. But just occasionally I'm flummoxed by my own tunes, like a slow reel I wrote for smallpipes (to be played with just drones) then tried to chord so I could play it on flute with a harmonic accompaniment. And, since what works with drones doesn't necessarily want to go with chords, I had a devil of a job to find both a 'fit' (in simple terms) and something that satisfied my ear (as logical, organic and
enhancing the tune). You could say the tune has a satisfying, logical chord structure in outlining mainly Em, G, A and D over A and E drones, yet take away the drones to substitute
just the 'obvious' chords and it doesn't work. It needs more. A digression, perhaps (as well as possibly what I deserved for trying to chord the only tune I can recall writing without simultaneously chording), but also an illustration of the potential dilemma even for well-tuned ears.
Something else I just have to chuck in here... see tune books published with added chords? (And I mean books of genuinely trad tunes rather than composer collections.) They're almost always wrong. Not, I think, because the editors don't have ears, but because they're working at speed to provide workable solutions (which aren't always workable!) for whole books. It typically takes time, and increasing familiarity over time, to arrive at truly satisfying, idiomatic and logical readings for all but the most blindingly obvious, but the chords given in such books are rarely going to be the products of such time and familiarity
with every tune. Most often they partially (or even largely) work, but spoil things by either glaring misfits or simply missing a trick or two. And surely the same's going to be the case for the vast majority of these miracle accompanists picking up stuff on first hearing!