What are they and how are they used?

I'm almost 80 years old and I have never heard of them. Please explain how they are used

I have an old machine from the 50s and one from the 90s. They have metal bobbin cases and metal bobbins, neither came with the spring. They sew perfectly without the spring. I did buy some washers from a very knowable woman on the 'net. A real waste of money. As was the slider thing.

You put them in the bobbin holder before you put your bobbin back and they are "supposed to make your sewing better" but in my machine they scrunched up and were no good. Guess that is what I get for not leaving well enough alone lol

I use them in my 15-91 Singer machine