When sizing, measure around the middle of the waist. Provides pain relief and compression for muscle injuries and general lower back dysfunctions; injuries to the lumbar discs and the sacro-illiac joint. Has 2 internal, flexible metal stays for support and adjustable elastic side straps. Captures body heat & increases blood flow. Allows skin to ventilate and release moisture. Size Small fits waist size (27.5 in. - 31.75 in.), Medium (32 in. - 35.5 in.), Large (35.75 in. 39 in.), X-Large (39.75 in. - 44 in.).