Boot Sizing: Measure around leg halfway between the knee and fetlock Small = under 7" Medium = 7" to 8" Large = 8" to 9" Extra Large = 9" to 10" The PRO H21 Thoroughbred Support Boot offers the same features as the H20 Sports Support Boot but is designed to fit the thoroughbred. These supports have been shown to reduce overuse injuries common in training race horses. They provide more support than traditional polo wraps and are more economical. The supports are machine washable. Sold in pairs. Available in small, medium and large in a variety of colors. Refer to size and color charts.