What do those numbers on spools of thread really mean? If there is one number, like 50 or 40, it’s the thread weight. If there are two numbers, like 50/3, the first is the thread weight and the second is the number of plies of thread. Thread weights are not consistent across brands, so use the numbers on the spool as a guideline when you choose thread for your projects, but trust your eyes and hands more. For an in depth look at sewing thread, download A Thread of Truth from YLI Corporation and clear up the mysteries!