Where do chord changes occur? Is it at the composer's discretion or are there any rules of thumb like it must change every bar, every measure, or every beat? This is a simple question it's no need to make the question any harder or more complex than it really is. I understand the theory behind basslines and how the notes are chosen. That's not what I'm asking. I'm asking are there any rules on when the "chosen note" should change. I'd imagine it changed with every chord change.
Chord changes are very much at the discretion of the composer and yes you do have various schools of thought on the matter. I personally am of the view that a conservative approach to chords changes indicates a large amount of maturity in composition.
You do have certain composers who changed chord a bit too often (For my liking at least.). Someone like Richard Strauss was notorious for his excessive chord changes.
It just becomes like porridge made with too little water, a little bit thick. The chord is never really given any chance to settle before it is changed again.
There is also considerations as to how many notes you would typically find in a bar. In 2/4 time it is very hard to have more than one chord in a bar as you simply have too few notes to do it. 4/4 time, on the other hand, you can manage it as there are enough notes to make a second chord clear.
You do sometimes wonder about whether certain notes in music are chordal or not but for the most part, you don't want ambiguity in regards to the harmony of your music. Remember that harmony is the foundation of music, if the harmony is not settled and clear you stop having music, you just have a collection of notes.
You will also have the chord changes be in line with the beats of the time signature. In 6/8 time seeing as you are still in duple time you will not have three chord changes in a bar. If you are in quadruple time you will also not have three chord changes. Chords should never change on the weak parts of beats.
That is why I think it is prudent when you start teaching harmony to get your students to make interesting music with the minimum of chord changes. Try something like making a melody, start on the tonic, go to the Sub-Dominant and then stay on the dominant chord for the rest of the phrase only having a cadence at the end.
It will teach you how to make music interesting in ways that go beyond simple chord changes.
Yes, it's at the composer's discretion, and no there are no Rules of Thumb. However, there are some commonalities in certain genres. Most pop music genres there is a chord change at the beginning of each new measure, with an occasional change mid-measure. In RAP and Hip-Hop, the changes tend to be farther apart, or sometimes not at all. I've heard a few that have no chord change at all thoughout an entire song.
In classical, especially before the Romantic era, the changes tend to be often, sometimes on every beat, sometimes twice per measure, sometimes more, sometimes less. With the advent of the Romantic era (say 1824-ish) chords tended to be held longer, sometimes over more than a measure, sometimes for several measures in a row.
As in so many aspects of music, the answer is: It depends!
OK, after reading some of the comments under the question, I should point out I thought you were asking about actual harmony changes rather than bass note changes. Some of what I said still applies though.
Where do chord changes occur? Is it at the composer's discretion or are there any rules of thumb like it must change every bar, every measure, or every beat?
As L38 says, it's at the composer's discretion - with a frequency of change that can range from changing not at all, every few measures, every measure, or every beat... or in theory, even faster, though that will be rare.
It is very common for the pace of change to vary throughout the song - faster chord changes are often found when the composer wants an emotional lift.
I'm asking are there any rules on when the "chosen note" should change. I'd imagine it changed with every chord change.
If by 'chosen note' you mean 'the bass note being played', then yes, it is quite likely that it will change when the chord changes. However,
- sometimes other instruments will change chord while the bass continues playing the same note. An example of this is the Pedal.
- On the other hand, the bass note may change at a pace much quicker than the chord changes. This may be because the bass is moving to other chord tones, or because it's playing passing tones.
Also, let me throw this in as a philosophical point: you can't always tell for certain what the chords are. With some songs, there aren't enough notes playing at the same time to identify chords; with other songs, there are lots of notes playing, but they might be moving in somewhat independent lines, not big 'blocks' corresponding to clear points in time where the 'chord' changes.
Another way of saying that is that 'chords' are only a concept - they don't exist unless the composer or the listener is thinking about them. That's a good reason to follow Ivan's suggestion of not considering any patterns you find to be 'rules' that you have to follow.