Refers to the amount of tension applied to the threads by the sewing machine, which can be adjusted. Many embroiderers use the 2/3 Rule meaning that if you look at the underside of the embroidery after it has been stitched, that you should see approximately 2/3 needle thread to 1/3 bobbin thread. Normally this is easy to do because most embroiderers use a white bobbin thread. Loosening or tightening the needle and bobbin tensions can accomplish this. Proper machine thread tension is critical to quality embroidery.