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I'm not using a shoulder rest and it feels ok > I do like the freedom of being able to move it for different scenarios. At this time I do put a small amount of pressure on the chin rest with my jaw bone when I am going down the neck otherwise my instrument seems to pull away from me. The upward lip on the chin rest and my jaw bone seem to hold it in place for me . I don't know if that is a progression that will eventually fade or if that is proper.
I've pretty much always played my acoustic without a shoulder rest. It is just comfortable like that, for me. For me, what works is a little more on the collarbone than what Pierre shows. As both StoneDog and Pierre say, a little bit of help from the jawbone for shifting, but much of the time when playing my jaw bone may not actually even be touching the instrument.
My electric, though, is whole different animal. LOL The different shape and greater weight make it not comfortable or steady at all without a shoulder rest. I don't feel that it is that my neck is super long so much as my electric is so thin and heavy and just balances in a different way than the acoustic does. It weighs about twice as much as my acoustic. It didn't come with a shoulder rest, and trying to play it without one for the first couple of weeks until I figured out it really needed one was very uncomfortable. Tension headaches if I'd play for very long at a time, and kinked neck on waking up several mornings until I made a shoulder rest for it.
Since I play my electric more on a typical day than my acoustic, I play more with shoulder rest than without. I enjoy playing both instruments, and don't find it difficult at all to switch back and forth between them.
But that's the thing, you use whatever it takes to make it work. The important thing is to be comfortable and to have the instrument steady.
I kind of disagree on the "long neck" logic for shoulder rests. While length of the person's neck is certainly one factor, I think it's a more complex thing than that, and the shape and location of a person's collarbone and shoulder and the shape of the jawbone are all factors. There is so much variation in how people are built (and sometimes some differences in instruments) that it is just sensible that for different people or instruments, one might or might not need to use a shoulder rest.
For all that I have seen some huge arguments in some other places about whether it is "best" to play with or without a shoulder rest, I feel that the only "best" that matters is what works good for an individual player and their instrument. But I also feel it is good to gave a fair try to both ways and see for yourself, rather than thinking one way or the other must always be better, and then trying to live with making that decision work. LOL
"This young wine may have a lot of tannins now, but in 5 or 10 years it is going to be spectacular, despite the fact that right now it tastes like crude oil. You know this is how it is supposed to taste at this stage of development." ~ Itzhak Perlman
I started out with a little fiddler pad, sort of like Pierre's sponge, but I think my neck is just too long. I couldn't hold the violin up without using my hand and my neck and shoulder would cramp up after a while. So I went and got a shoulder rest and am much more comfortable. I was a little bummed though, because I like the idea of the freedom not using one would give. Maybe once I get better I can try it again.