I thought Larry wrote a really informative piece, but I wondered about two points:
First, he mentioned that Jovino Santos Neto (who really does qualify as an expert) had said that early choro composers modeled the structure of their compositions on 17th-century classical music. Since the early choro composers (e.g., Calado, Gonzaga, Nazareth) were writing in the mid-1800s, this didn't sound quite right to me. Surely, their models would have been music as played in their own time: European classical music of the 18th and early 19th centuries, as well as dance and salon music from the same period.
To check on this question, I emailed Jovino, who graciously sent me a reply. He said that what he had meant was that "what comes from the 17th century...is the rondo form of 3-part music...In its beginnings as the urban music of Rio in the mid-19th century, choro was basically a syncopated way to play the European music that was in vogue at the time: polkas, mazurkas, waltzes."
That sounded reasonable to me. But that brings me to my second question:
European rondo form (ABACA or some variant) is often cited as the origin of standard choro form (AABBACCA or variant). Again, I'm no expert, but that seemed open to question. The polkas and marches that I am familiar with have a basic form along the lines of AABBA - trio - A (or variant). The repeats are significant; polkas seem to be a more likely model than classical rondos.
Searching the web, I found this paper by Marcos Mesquita (if you download it, scroll way down to where the paper begins).
Mesquita argues that the AABBACCA choro form probably derives from European dance forms that had a “trio” section (that would be the “CC"), such as polkas and minuets. Polkas had repeats of AA, BB, and CC. Repeats are found in polkas and minuets, but are not part of the classical rondo form. Here's an excerpt from the paper:
We must point that: 1) Traditional choro form with its repeat signs is:||: A :||: B :|| A ||: C :|| A || – the recapitulations of A section after B and C areplayed by indicating da capo; 2) No rondo form has repeat signs in each section...He points out that some pieces by early choro composers had a section designated as a "trio," in the CC position. Over time, the “trio” designation was dropped, but the form stayed the same.
This makes sense to me!
Interestingly, as Larry mentioned in his writeup, ragtime tends to have a form similar to choro. Scott Joplin's Maple Leaf Rag, for example, is AABBAACCD. Not exactly the same as choro, but similar. Joplin's Euphonic Sounds is AABBACCA, the same as the common choro form. Here's a page from a book on Joplin that makes the same point that Mesquita does, but with regard to ragtime - that is, some music historians see rondo as an antecedent form, but it's unlikely because of the lack of transitions, and the presence of repeats. Ragtime is often mentioned as having its roots in the march; many marches begin with AABB - trio.
There is no way that ragtime could have influenced early choro, as ragtime dates from later in the 19th century. It's also very unlikely that choro influenced ragtime. Yet both genres developed similar structures, derived from European forms.
It's likely, though, that ragtime (and jazz) influenced later choro composers like Pixinguinha, starting in the 1910s-1920s, when ragtime achieved popularity, and was available in both sheet music and recorded form. The influence of jazz continued into the bebop years, and in fact to the present.
But again, I'm no expert. Comments are welcome!