I use regular thread in mine when I am sewing. but for embroidery I use regular embroidery thread. they fit perfectly. use the little orange round plastic thingie on top of the threads to keep them from locking up if the thread gets into the slot on the top edge of the spool. but if you are using embroidery thread, you cannot usually get the orange circle thing on. but if it fits, use it. serger thread works ok for sewing, but not for embroidering. a lady at a fabric shop told me yesterday that it was not good to sew quilts with serger thread as it cuts through the material. don't know if she was spinning me a line of bull or not? anybody else know? would love to find out. i do know that there are some threads for hand quilting and others for machine quilting. but if you buy it at a reputable shop they will advise you on your threads. hope this helps? :roll: