The double drive wheel is named after its drive band, which goes around the spinning wheel twice. The drive band turns the flyer, which is the horse-shoe shaped piece of wood surrounding the bobbin, as well as the bobbin. Due to a difference in the size of the whorls (the round pieces or pulleys around which the drive band runs) the bobbin whorl, which has a smaller radius than the flyer whorl, turns slightly faster. Thus both the flyer and bobbin rotate to twist the yarn, and the difference in speed continually winds the yarn onto the bobbin. Generally the speed difference or "ratio" is adjusted by the size of the whorls and the tension of the drive band.