Nika - I remade Clipped Wings AGAIN.
if you're ever on, here it is- it is far from being done, I got the introduction started, though
I worked hard to formulate and carefully articulate a process to match or write a bass line to a melody, and the content of my post advances the discussion without doubt. Although I want to make sure to get credit where credit is due for what I’m sharing in the post, I do see your point. So I’ll repost or edit my answer with the website URL removed from the graphic and post.
I agree with Williamson in that it depends on the music style. And Timaeus provides a useful idea in presenting common intervals to use. At the most basic level, I find the bass notes to go with the melodic notes played on (or sometimes immediately after) the main downbeats by playing the notes that are at one, three, or five intervals below the melodic note. I can further narrow down these three options by following this melody to bass note assignment:
Play 1 or 5 interval below melodic note if melodic note is note I, 5 for II, 3 for III, 1 for IV, 5 for V, 3 for VI, 3 for VII. The gray colored interval numbers are those that occur less frequently. The diagram below provides a visual to help map the melodic note to corresponding bass note identified by the number of intervals below the melodic note. These intervals work for all major scales (not just C) and I think minor scales with slight changes in the note intervals.
Would like to hear other’s thoughts on this process.