Okay, you’re strumming a guitar trying to find the chords to a familiar melody. You’re not shooting for anything unique or creative, if it only takes I, I-V and V that’s great.
What makes one chord right and another chord wrong?
Is it possible for a person well versed in music theory to look at a single note melody transcription (of a song he’s never heard before) and guess (or know) where the chord changes would occur… and what the chords would be?
Would that talent extend to more unusual chords… “hmmm, looks like a ninth would sound good there”?
It’s called implied harmony. It’s not an exact science, but in a melody using tonal language, i.e. major/minor key systems, there’s inevitably points which just beg for particular harmonies. An extreme example is this piece of Bach, where only one note sounds at a time, but it also explicitly outlining particular chords. As to whether more ‘unusual’ chords are involved, that depends on the genre in question - I could happily scatter diminished 7ths and neopolitan 2nds around if you wanted it to sound vaguely classical, but I’d struggle to provide a convincing jazz harmonisation, while for other people the opposite would be the case.
How? It’s about a familiarity with the harmonic system in question, because melodic structures are completely interdependent with this aspect of music.
Given a single note melody? I’d say no. Any given melody can support a wide range of possible chord changes. Now, given a melody, a genre, a time period, etc., a reasonable guess might be made to what chords would go underneath, but there’s no guarantee those are the “right” chords, i.e. the chords the composer scored. Melodies are reharmonized all the time, especially in jazz.
Note that in the first 3 bars the only notes that are used are Bb, D, F… and these are the notes that make up a Bb major chord - no surprise that that’s the chord used for the harmony. At bar 4 we see Eb, C, G… these suggest C minor and that would work, but F7 (F, A, C, Eb) is better. It keeps the bright Major key tonality and F7 is the “Dominant” of Bb which is a strong and fundamental harmonic relationship in Western Music (and it’s one of the three chords in a three-chord song). The G in the melody is a “nonharmonic” tone (in the context of an F7 chord) adding a bit of spice and tension in the melody - you can feel it straining up there, fighting against gravity trying to pull it down to an F.
At the most basic level, a single note melody would give you a note inventory, the note inventory would give you a probable key, and the key would dictate the inventory of chords which would match the key.
To simplify it a little more, the melody would give you a scale, and the “proper” chords would then be the ones that fit the scale.
None of that would give you a necessary progression of chords, but you could fairly easy come up with a framework that would at least be functional and not sound wrong.
The notes which tend to sound “right or wrong” for a particular chord tend to be the ones that are the most consonant with it (i.e. they match the notes in the triad).
To see how fluid things in terms of harmony, one need only look to the myriad of guitar tabbing sites out there, which always seem to contain wrong chords all over the place. The chords may work (say, substituting an F major where a D minor or A minor is called for), but they’re not the right ones (as in, what’s on the recording).
It’s pretty easy to come up with a harmony that works for any given melody. There are certain chord progressions and harmonic resolutions that consistently work in Western harmony. That said, it’s also easy to come up with a very boring chord progression that way.
As said above, what you generally look for is consonant notes: chords that share common notes with the melody. That’s why in a misnotated D minor chord, an F chord may work, as F is F-A-C, while D minor is D-F-A. Sometimes, you can look at it and see it doesn’t look right, given the key and the chords that are usually part of the key. For example, you’re probably not going to have B flat (Bb, D, F) going to G7 (G-B-D-F) in the key of C (then again, you might), but a D minor (D,F,A) or F (F,A,C) may work equally well). Once you get into jazz harmonies, anything goes. Where you have that G7 in typical Western music, it’s not unusual to find a Db7 (Db,F,Ab,B), and that’s a chord you generally would not expect in classical harmony. That’s why I state that the genre, time period, and type of music is important in making a guess as to the harmony, but that’s a guess at best. Lots of chords will work well under any given melody.
For example, if you take the melody of Smells Like Teen Spirit, and gave it to a musician that’s never heard it before, I’d be curious to see what harmonies come up. That song does not follow a typical classical chord progression (the song is in the key of F, and there no C7 chord in it) and is scored as F5-Bb5-Ab5-Db5. However, who’s to say the chords aren’t F5-Db5-Ab5-Gb5? They work, too (unless I have the melody wrong). Or how do I know the intention is not a droney, dark, metal-ish F5-F5-Gb5-Gb5? If I’m familiar with Kurt Cobain’s work, I might be able to guess F5-Bb5-Ab5-Gb5, but it’s a guess, at best. It could very well be F5-Bb5-Ab5-Gb5 (which makes sense, too.)
This is actually an exercise I give my students - I call it ‘Grandma’s Harp’. Make up whatever scenario you like; the one I use is that you have to play at a wedding reception, and Grandma wants to play some song she knows on the harmonica. She doesn’t speak any language you can identify, you’ve never heard this melody before and you can’t even guess what country it’s from. It’s not written down, and you have to accompany her - she can’t change anything she’s doing, so it’s down to you to adapt, accommodate and accompany. There are no right answers or wrong answers - play what you think sounds good.
I then give them a recording to take home for the week of some oddball melody that I’ve just made up - they play along with it and try to sort something that makes it sound better without intruding on Grandma’s playing. The better they get at it, the weirder I make Grandma’s tune. It all helps them to develop their ears and get a sense of what their personal taste should become…
