Two Stretch needles mounted on one shank create two rows of stitches simultaneously.
Twin needles come in a variety of needle types and sizes. Double needles are actually two needles mounted on one shaft and are used to create two rows of stitches at the same time. When using a double needle two spools of thread will be used in the top of the machine. Pass the thread from each one on a different side of the tension discs and thread one into each needle. Only one bobbin thread will be used and the stitches will create a double "web" of stitches on the back of the work. Check to see how you like this under stitching prior to using these needles.