This question bugs me since I started writing music. I'm asking in general, but I'll show an example so it would be more clear. Keep in mind the notation is arbitrary, as I don't quite know what I'm doing.
This is an 8-bar excerpt from the piece I'm working on currently. Judging by the notes that fall on the strong beats I would guess this is somewhat E-minor-ish. The chromatic rundown skips A natural even though it is present in the first bar. So the question is does this have a chord progression, or is it just Em throughout? I can't tell. But there is another voice.
This one is much simpler. It reinforces A natural, but it also has Bb (or should it be A#). Which would imply a flat fifth in the scale. But it still fits into the E-minor-ish sound. But there is a third voice.
Here you have F# and G which are again notes of E-minor, the problem arises when the upper F# becomes natural. With another voice playing E at the same time this would make it a cluster of 4 semitones in a row (E F F# G). And if this is indeed E-minor that would make it so it has both natural and flattened second degree playing at the same time. But with this third voice added I feel like there appears an implication of a chord change when F# becomes natural.
Now the problem is that I have to write a bass line and without knowing what kind of harmony I'm dealing with it became an impossible task. I tried treating this as E-minor and it didn't work well. Sometimes I felt like this is centered in A-minor, but nothing came out of that too. So unless someone opens my eyes I feel like I'm stuck. So if you have any ideas I would love to hear them.