WonderFil pre-wound bobbins are wound with 80wt DecoBob thread in a reusable plastic bobbin, which means there is no glue or wax holding it together (which also makes for happy machines)! DecoBob makes for an ideal bobbin thread as using a finer weight thread will also minimize the bulk in whatever you are doing, however DecoBob also has the strength, versatility, and reliability you can count on.