I'd say 'thin' enough, really. But, for the life of me, I can't recall how thin they get.
Also, it won't really work for anything but the outside strings. So, now that I think about it, you should probably go with the credit card or fairly thin guitar pick. You can check individual strings that way.
Why won't it work for all the strings, the gap set I have is thin strips of metal 2 1/2 inches long and 1/2 inch wide at different thicknesses on a key ring kind of thing
Oh, that'd probably work then. Those are pretty much feeler gauges. I was picturing the short ones where they're round and not very big.
I think I have one of those too but I was thinking about the what you called feeler gauges cause that's what I use them for. I don't know where they are, I went and looked for them and they were not in the toolbox. My son have taken them and put them in his toolbox
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