Yeah, in essence keeping the instruments separate in your mental models. When I go back to 4ths tuning I'm sorta able to slide back into old fingers. 5ths does not transfer to 4ths for me. Only certain 4ths things transfers to 5ths. The boxed positions in 4ths shows up as a diagonal on 5ths for example. You could do the same in 5ths but i never made that mental switch. I've seen links to what people are calling a BACE (Bass Cello) ... aside from some greats making records that I've seen I think they're losing out on all the potential of what cello tuning has to offer and you def know what i'm talking about.Interesting discussion. I’m studying cello now. 1. I love that so many open ringing melodies can be made in 5ths. 2. Haven’t had really to many problems keeping them straight. I use German bow on the bass and French on the cello, making it simpler to keep them separate in my mind. Because I have smaller hands, the cello is better for making melodies but I still am working out walking bass lines on it. One thing I’m not doing is trying establish patterns on the cello but to actually think of the actual notes making up the changes in a tune as well as setting up in my mind the keys and modes. This helps with the 5ths problem after 45 years of 4ths on the bass. Also the cello really is designed to play melodies that help to outline the piece of music as well as solo cello pieces that really ring in certain octaves. Not sure if this really is because of the 5ths thing or just the way it’s been written for over the centuries. Just my take as I progress so far.
From what I read on Joel Quarrington's website that basses originally started with tuning in 5ths and the tuning in 4ths came much later mainly due to the role of the bass as a support instrument and learning all the shifting was hard. Lets face it, even double bassists as soloist artist is a relatively new thing - at least not a norm until pretty much now.For melody capability, 5ths inherits a lot of music from violins, violas, cellos, mandolin, mandola, and a myriad of other instruments that draw all or part of their tunings from the violin.
4ths inherits a lot of backup/harmony capability from guitars ("the last 4") and other similarly tuned instruments.
What double bass does not inherit from any of those instruments is handy size, which is probably why the 4ths tuning solution has evolved through the centuries as prominent for the double bass. But that doesn't mean 5ths cannot be done on double bass, just that it has to be approached differently, as most of our 5ths practitioners have so well described here in this thread.
On these smaller normally 5ths tuned instruments, there are a few among us who do use non-standard tunings. Violin in particular frequently can be found tuned for drones or drop-tunings in certain genre. While it's rare, a few of us also use non-standard tunings for mandolins.
As someone who frequently uses non-standard tunings for instruments other than double bass, I'd opine that at least in my case it's mostly for convenience. But where that convenience disappears is if / when a person needs training or learning materials, as there is not much of a likelihood of finding a teacher or a reference document for non-standard tunings. A person who wonders into that rabbit hole needs to be extremely autodidactic.