I took a class once from a Superior rep and she said that Bottom Line is a poly thread and very slippery besides being a smaller diameter. She would use a larger diameter cotton thread in the top like King Tut. She also used a size 12 needle. I tried Bottom Line on both top and bottom and had a horrible mess....Janome 6500. Switched the top thread to Aurifil which is a 50 weight cotton and everything was fine. It's worth the effort to get the tension right just to have the larger throat space in my mind.