I have watched at least 6 music theory videos on how to harmonize a melody in 4-part writing and none of them start with the bassline first. All of the teachers seem to look at the notes of the melody (usually the soprano line) and then figure out which chords might work with the notes by selecting harmony from a diatonic pool of triadic chords. Before getting to 4-part harmony, I had been studying counterpoint and I realized that when given a soprano part or any other part, it seems to make more sense to first write the bassline using counterpoint and then add the other voices in between. What are the advantages (and disadvantages) of harmonizing a melody by writing chords first, rather than the bassline first?
In 4-part writing, one of your goals is to create 4 independent voices, but adapting the priority of each voice dynamically throughout the piece can be a way to guide the listener's ear and add melodic interest.
By writing a section of music with one voice (or pair of voices) before the others, you are mentally giving priority to that voice (even if it isn't necessarily the main voice), and making other elements of the composition, such as the harmony, more or less dependent on supporting that voice.
...wouldn't it make more sense to first write the bassline using counterpoint and then add the other voices in between?
A lot of times the soprano and bass get priority because they are the easiest for the listener to hear, and they are very independent by nature, because they are so far apart; but, sometimes you might want to start a section with an inner-voice just to make things more interesting.
What might the difference be in the end...?
Let's look at some pieces! Here's an example from Aldwell-Schachter:
It's hard to say exactly what the composer's goals were when writing this piece of music, but there was definitely some thought put into the treatment of each voice. In the first measure, do you see that wonderful bass line holding through the
iv-iiø6/5-V4/2 then resolving down in the
i6/3? The bass was probably one of the first lines written in this measure.
Here's another example of strong bass motion taking priority over other voices:
In this example, even the harmony is largely dependent on making this bass line work. Here's a real-world example:
In Ach bleib bei uns, Herr Jesu Christ (Bach) there's a really interesting motion in an inner-voice.
Do you see that beautiful descending line in the tenor in measures 6-8? You can hear an audio recording at:
The treatment of the tenor line is very goal oriented, whereas the alto is taking on a more supportive role. This leads me to think that Bach probably wrote the tenor before the alto in this section—the alto is dependent on the other voices.
...why would one approach be better than the other?
You might want to base a section of music on a melody you heard in your head and give it to the lead voice: the soprano. Other times, you might feel like the soprano's taking too much attention and write a strong alto line first to let it direct the other voices. Each technique is just another tool in your toolbox.
They're all different faces of the same thing. If you start with soprano and bass, then for the most part, it will be pretty obvious which chords you'll want. Similarly, if you start with a harmonic motion, then your choices in bass will be fairly limited.
The reality is that in addition to a melody, you will have a few motifs that come to mind, and then you'll have to figure out which ones are worth realizing. For example, if I find that my bass has moved stepwise a couple times in a row, I might ask "Hmmmmm. . . can I run this bass all the way down to the dominant?"
Or, I might find that I can harmonize the melody by a 3rd or 6th in the bass, and think: "Okay, could I actually make this fully parallel, and let that new idea become an important part of this section?"
You get the idea-- you have a few things to juggle, and how you prioritize them has to remain highly organic, and depends on which motifs you want to fully realize.
If you compose a bass line for pure accompaniment, whereby you just choose a bass note that sounds nice with the melody at that point, there may be no difference at all: the bass line, though chosen first, might not make any more sense as a melody on its own than if you figured out the bass note by dropping it down from chords.
Unless you're treating the bass as a melodic voice whose lines are supposed to draw the listener's attention, then it probably doesn't matter, as far as the end result, whether you harmonize bottom up or top down.
I don't think it has to be an either/or. A bass instrument can accompany through most of the music, but have some solo sections, or brief passages where that instrument plays a run of faster notes that catch the attention. Those passages could be composed bass-first, perhaps with some additional attention to what that instrument is doing in the neighboring measures, so as to transition into and out of those parts well.
The way I look at it is the bass and soprano are usually more important than the inner voices of tenor and alto. So given a melody to harmonize write the bass first and make it good, because the inner voices that complete the harmony are less consequential.
Consider harmonizing a simple line of
SOL LA SOL...
...if we add a bass to that - a clear, tonal bass of
^1 ^4 ^5 - we have incomplete, but unambiguous
V on the first and third chords. But the middle chord is just a harmonic third, an incomplete triad which is ambiguous.
When a third line is added we can get a complete
I, and a harmonic third for the
V, and for the middle chord we can get either a
IV or a
ii6 chord. But the choice between those two chords is not of huge consequence. The soprano and bass are unchanged, and the middle chord is a subdominant type regardless of whether it is specifically
ii6. It's in this way you can say that from a tonal perspective a good bass is more important that specific details about chords.
That doesn't mean you necessarily need to add the bass first, but it explains the basic idea behind why the bass is so important.
In any genre where the bassline is often hard to hear or obscured (e.g. classical music, soundtrack music, often light music, some metal music if we believe Metalocalypse's Dethklok), harmonizing a melody with the bassline first makes no sense comparing to harmonizing it with the chord progression first. Even in genres with an easier-to-hear bassline (e.g. ragtime, marches), if you expect to hear clear chords in the accompaniment instead of single notes in a bassline, harmonizing it with the chord progression first is sensible.
It's mostly youtube bias - no youber expects the audience of a "harmonizing melody for greenhorns" video to know anything about counterpoint. Actually, I think most of them don't know much about it either.
Also, before you are quite advanced (i.e. far beyond the introductory video), your choice of chords and positions is very limited and it's easy to write a bass line which is impossible to voice. So, before you learn to think about position, bass and the harmony as one thing with multiple facets - the simplest way to write is to get the chords right.