The shaft of a boot is the part that covers your ankle and calf, running from the top of your arch to the collar of the boot. It is the tall, cylindrical component that extends up your ankle and is measured from the arch to the top of the shaft. The primary purpose of the boot shaft is to provide a protective shield for the legs. One of the biggest functions of the boot shaft is to add stability to the ankle, making it less likely that you might sprain or otherwise injure your ankle. A boot with a snug shaft will help keep the foot in place, while overly tight shafts can mean that your normal shoe size will be too small. When measuring the height of your shafts, start at the arch, not at the sole. The circumference should be about half an inch wider than your leg, and this can be adjusted using leather stretching spray or a boot stretcher.