Generally, anything Bruch and above is considered advanced in the overall repertoire, considering the early Twinkles all the way through the hardest concertos. I would consider intermediate Vivaldi a minor through Kabalevsky or DeBeriot roughly.
However, many people often refer to the concerto repertoire in another way. Beginning would be pieces like Vivaldi, Haydn, and all the way to Kabalevsky. Bruch, Lalo, Saint Saens, and even Vieuxtemps are intermediate. Any of the big warhorses (and modern concertos) like Tchaikovsky, Beethoven, Sibelius, or Shostakovich are advanced.
Those categories are market-force-driven and arbitrary to some extent (in language classes they will invent as many such categories as necessary to distribute the numerous applicants and their money among the teachers available), and if you think of yourself as being in such a category it's like making yourself think you are in a rut when you aren't. If you simply improve every month linearly you'll reach the heights without having to assign yourself to a self-condemning category. This is why the ABRSM can be useful - grades go "linearly" from 1-8 but you can regard associate diploma level as grade 9 (especially if you go up a grade a year, since the associate diploma seems to come about a year after grade 8) and then imagine a grade 10 above that. In fact some methods have grade 10 and above. Technically ABRSM rates something like grades 5-7 as intermediate and everything above as advanced, but I'd suggest that was irrelevant if it was getting you down.
(I often see this question asked in language/literature forums. I have a tongue-in-cheek definition I invented - you are a beginner if you spend most of your time asking, what's the author saying? You are advanced if you spend most of your time asking, why is the author saying it?)
To Gordon's point, it's about how you play whatever you play, and that's something that can't really be disguised, even if I'm somewhat cynical about the credulity of most audiences.
I'm probably wrong but for me the Beethoven concerto is much more playable than those of Saint Saens and Vieuxtemps.
We can of course say that it is more demanding in terms of "musicality". (even if many examples of recordings by very young violinists prove that musical genius does not wait for age..)
How about we define (late) beginner, intermediate and advanced by how someone plays a piece common to all rather than by the technical difficulty of the piece? For example, listen to Itzhak Perlman (Concertos from my childhood) play the Andante from the Violin Concerto in B Minor, Op. 35.
IMO defining playing levels by concerto might be very useful to the teaching process - but it is not useful at all to the business end, and that is the listening one. I have heard virtuosically-technically brilliant violinists who could play the whole gamut that make me want to run screaming from the auditorium. And I have heard the opposite, ones that played simple songs and brought me to tears with subtle nuance and might-bending projection. Which one of these is 'advanced' and a 'beginner' in the ears of the audience - which we here too often forget is the actual purpose of playing an instrument.
For me this is what advanced is all about, beautiful playing that people want to listen to.
That suggests that the performance determines the stage. And here's the real kicker: if you want to reach the next level in your playing, make sure your teacher is performing at that stage.
So how should we define this? Should we define your skill level to be the level that you could perform well, or your peak skill level. I think I have the ability to perform the Mozart 5th Concerto, quite well, in a way that others would enjoy. Am I advanced? I have no idea. Would you enjoy my performance of the Bruch violin Concerto? Probably not. However, I could work to the point where it was reasonably accurate.
And let's be honest with ourselves: if we are going to define our skill level as the level that we can perform well, then the majority of us would have to admit, we are simply intermediate.
The use of "beginner", "intermediate" and "advanced" is almost always in the context of pedagogical reference. So it's kind of pointless to define "advanced" as equivalent to "professional". Professional is professional (although there are certainly pros that cannot play advanced-level repertoire, i.e. not capable of technically executing a listenable Bruch, much less a beautiful one).
Some students certainly "advance without improving" -- i.e. they play harder and harder repertoire without fundamentally improving their technical foundation. This can get players into a house-of-cards situation where they can theoretically read the notes of advanced-level works but the poor technical foundations are obvious.
(Some of you may remember a parent who posted their son's Bruch recording some time ago -- this was a clear example of advanced repertoire on a shaky foundation, and ultimately the parent took the advice of commenters, talked to the teacher, and the kid was put through a technical reboot.)
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