Mark said
I personally have no other choice but to play left handed, I had a accident at age 10 with a table saw that amputated my index finger and severally mangled my middle and ring finger it is  impossible to note with my left hand. After the accident I switched my acoustic guitar to be strung up left handed at my teachers request, the issue is unless I take my left handed guitar I can't play anyone else's guitar, i never been to a jam and meet another left handed guitar player and the selection is slim even to day if you look for one. At 15 I began learning the electric bass and I decided to just play a normal right handed bass left handed no issues encountered in doing this. At 23 I bought nice old right handed fiddle  for the same reason I learned the bass with a standard right handed bass I can play anyone's fiddle or bass, then i took violin lessons for 3 months then my second child was born between 2 small kids, school, work, playing and singing in a group along with a couple of other hobbies I slid the fiddle under the bed for 30 years. I just passed the 3 year mark of being back at playing the fiddle so far I have found nothing that I can not do on the fiddle that a right handed player can do, at my level anyway. Will I ever be a Pearlman or Perrie Holestine, no I won't, I know that and accept that, but to play with good tone, reasonably in tune and with good rhythm I believe is doable and is what I'm working towards daily taking each step one day at a time.

Mark  

Respect, considering all your experiences! Are you still also playing guitar? I mean, you potentially have the option to make a recording of guitar chords and use that as play-along. And if you have a good ear for chords you have lot's of options to be creative on the violin as well. I don't expect to become a virtuoso either—1600s music isn't outspokenly virtuosic anyway. I don't consider virtuosity healthy anyway, like any kind of extreme sport.

What I'm working on are Bb scales within improvisations. It is easy to play pieces in E minor or A minor. D minor gets already harder because you often start with Bb on the first finger and then continue with C and D. In G minor Eb adds on which makes the scale harder still. Plus you now mostly start with Eb on the first finger of the D string. Which isn't a great problem but all this together tires me out soon. If you proceed to C minor you have even three ♭ which means: all those difficulties appear more frequently plus you often have Ab on the G string. That's why I don't play C minor at all—that will have to wait..... 

I played trombone in the 80s and didn't know anything about Aebersold play-alongs. I was performing a lot at that time, with a swing combo—at times even 3 times a weekend. Of course this routine made me better, but now I must say, nothing got me so much ahead than Aebersold play-alongs. Every day you have your personal professional rhythm section at home and play like on a jam session. And if you split the stereo channels, you have the piano in one speaker and the bass in the other. I closed my eyes and thought, "Wow, this feels really like the guys are here in the kitchen and we're having a session."

But of course  the most basic thing is listening a lot to the music you wanna play, to gather ideas what to play. Improvisation is playing out of a huge pool of hearing experience. Your brain cuts the phrases you've heard into pieces and then you can creatively play with that.

I have a weird problem. There's a basic rule in jazz: "The beat is always written in quarter notes." I learned that from books about jazz since I was 15 years old. The walking bass of a swing tune will principally be written in quarter notes. The old masters certainly don't care about that and so I have to transcript this here too.

Source: Screenshot from IMSLP357195-PMLP100008-Bach.pdf (this is no link, just blue script, underlined)

Fiddlerman said
I'm sorry, but I don't understand your problem. Please explain.  

I will have to make 2 bars out of each bar and and all the eighth notes in the bass line must become quarter notes. Because I instinctively conceive the beat as quarter notes since age 15 when I read the first books about jazz. So if I have eighths in above walking bass it will confuse me and make it extremely difficult to read and play the notes. Bach really wrote a very swinging, pulsing walking bass here and what he did was not wrong at all. But in jazz I've never ever seen a walking bass written in eighths. The rule has always been: the beat is in four quarter notes. I don't see how I can retrain myself after over 40 years, it won't work. But whatever, I will transpose to C major anyhow. And it will be a big headache since writing and figuring notes always gives me pain, lots of erasing correcting and sighing.....

In the left-handed playing issue I came to the result this shifts me on the clock of keys.

B♭ major has always been a key of question, it always was very common in jazz but it's exhausting and violins tend  to ♯-keys anyhow (because the fingering is easier) and here's the result I came to:

The consequence of playing a right-handed violin left-handed means shifting one key further towards the ♯-keys in the preference of comfortable fingering. I now cancel B♭ major but keep its relative g minor. The problem in ♭-keys mainly starts on the A string. The first finger hits B♭ and then goes on with 2 whole tones. That's already exhausting but if you finally end with a 4th finger it's very hard. My right fingerboard hand  tires soon, especially as B♭ is very frequent as keynote, plus E♭ on top. E♭ on the D string is not too hard but as it adds on I tire out sooner. Right-handed players are closer to the A string, so they don't suffer so much. G minor is not that tough because it is connected to many ♯-chords where the fingerboard hand can recover.

Right-handers have the same problem in E♭ major on the D string, but this is not so striking because the classical violin mostly uses the A string and touches the lower notes of the D string not so frequently. But generally the E♭ major difficulties right-handers have on the low E strings starts for left-handers in B♭ major on the A string.

My conclusion now is: Mostly avoiding the A string in ♭-keys, especially in g minor. Secondly I should get more interested in ♯-keys since they are easier for me than for right-handed players:

D major + its relative b minor, A major and E major are new to me on the violin, but I tried them today and it works fine. I always assumed A major would be difficult but that's not the case. I'm not used to play on keyboard instruments in these keys and I won't (because it will only sound ridiculously) but guitar players prefer these keys too and I'm planning to work with a guitar player. So that's good. In jazz piano I always tended to ♭-keys because they are common in jazz—mostly because wind instruments like them. On the trumpet (yellow circel) C major, F major, B♭ major, E♭ major and A♭ major are very common in jazz, but G and D♭ major happen at times and I was used to jam with professionals and be ready for all of them (on sessions you will rarely be confronted with G major though). So that's why my trumpet routine has this wide range. Recorders are centered around C major—G♯ major, C major and F major are very comfortable in fingering, G♯ more than F. But playing in A and E major on recorder is very difficult.

AndrewH said
I started out finding G, D, and A major easiest, but once I reached a certain level, my preferences changed dramatically and I started finding keys with flats in their key signatures easier to play than keys with sharps. That's because sharp keys call for a big 4th finger stretch in 3rd position. I rarely use 4th finger for the natural note in 1st position, because I have small hands and find it easier to shift and use the 2nd finger. (In fact, shifting to 3rd position has become so automatic that I now have to write in open strings where they are preferable, to remind myself not to shift.) So now I find that E-flat major / C minor is the easiest key signature, and A major is relatively difficult. My range of preferred major keys now goes from A-flat to D.  

Interesting to hear that you also avoid the 4th finger on natural notes. You do it by changing position whereas I do it by always choosing the open string. I had a heated debate with my ex-teacher and then exercised my authority as the paying part. I did play higher positions rather unaimed in summer to make my hands stronger (which helped a lot to feel more comfortable and easier in my regular 1. position technique). It was actually the same training methods like runners training with weights to feel like a feather without in competitions. But I have no idea what happens in the 3rd postion in A major.

In A major I have just one whole tone step (B and C♯) on the A string, then the half tone step to D is quite a comfort to my 3rd finger—after that follows the open E. Whereas in B♭ major I have 2 whole tone steps (which are already quite a stretch) and then E♭ on the 4th finger on top. That reminds me a bit of a medieval torture rack. After that I have to try to get the note F on the E string as quick as possible out of this terrible position (it mostly goes wrong). I can't play anything nice on the E string after that intense tension!

Well, on the D string I start with E and then have two whole ton steps in A major, but after that no 4th finger since my A is the open string. The only difficulty is D♯ on the 1st finger, which is a shift down: It is difficult to hit a clean C♯ from there but time will heal that.

In a way my A major scale is half a shift of position: The 3rd finger hits the D string G♯. Previously, I prohibited my 3rd finger from leaving his G place on the D string. Which was a good thing to teach him precision on natural notes. Now it's  time for more flexibility.

So ♯-keys are comfortable to me because they have many options to involve open string notes. F major has still lots of options to involve them, but if you go on with B♭, E♭, A♭ they get rarer and rarer. These keys are comfortable for brass instruments because there you have lots of natural notes where you don't push down a valve. They also sound more open, similar to open strings.