Now I know length is part of the answer. Nobody wouldn't consider a 10 minute long rondo to be a bagatelle. Bagatelles tend to be around 3-5 minutes in length. Also, from my experience(though this is biased towards Fur Elise and Bagatelle in C minor(the one at a presto tempo) because those are the 2 bagatelles that I am most familiar with), they seem to follow one of 2 forms most often. Those being:
Rondo form, usually ABACA instead of the more complicated rondos you see at the ends of sonatas such as ABACABA. This is how come, despite the short length, Rondo Alla Turka does not count as a bagatelle, its rondo form is more complicated than a simple ABACA and is much closer to Sonata form in its nature(that and it is the end of a Mozart sonata, just more often played by itself than with the whole sonata)
This is the Sonata-Rondo form that Rondo Alla Turka is in. You wouldn't expect to find this complicated of a rondo in a bagatelle.
The other form I most commonly see bagatelles in besides Rondo form is:
Ternary form, sometimes itself split into Ternary form subsections, especially in the A section.
An example of a Bagatelle in Rondo form:
The most famous bagatelle in existence
An example of a Bagatelle in Ternary form:
This one has an A section that itself is in ternary form. In that sense, it is closer to a rondo, but I'm still counting it as ternary form because of the Scherzo and Trio structure of the bagatelle as a whole.
Another thing that I notice, at least in minor key bagatelles, is that there will often be 3 sections differing by key and intensity. One of the sections will be intense and in the minor tonic. Another section that is also in the minor tonic will have a more relaxed feel to it. And then there is a section in a contrasting major key. This key is up to the composer but I usually only see a few options used, those being:
- Relative major
- Submediant(this is the case with Fur Elise)
- Subdominant major
- Parallel major(this is the case with the C minor bagatelle)
But obviously form and length aren't the whole story, otherwise a lot of short, simple rondos would be called bagatelles. And granted there are a lot of bagatelles out there(especially Beethoven bagatelles), but the majority of short rondos aren't bagatelles, just rondos.
So besides the piece being in either Rondo or Ternary form, being short, having 2 sections in the same key and another section in a contrasting key, and having the 2 sections that share a key differ in intensity, what makes it a bagatelle? Does it have to do with being in triple meter as opposed to 4/4 or 2/4? No, because some bagatelles are in 6/8 and that isn't triple meter(well, I guess it can be either duple or triple depending on context but usually 6/8 is duple meter with a triplet feel, not triple meter with a straight eighths feel).
So, I have found a lot of similarities amongst bagatelles in their form, key, and length, time signature types differ, but I don't seem to have found what makes a piece a bagatelle. So what is it that makes a piece a bagatelle?