So Fine! #50 is a lint-free, extra smooth #50/3 filament polyester.
Here at Quilts on the Corner we love this in our bobbin while using Magnifico on the top. It is "so fine" that we can more yards on a bobbin.
I love So Fine thread by Superior. My two favorite colors are #402 and 456. I use those two colors for all my piecing. This color was for my mom.