SHOE MUST HAVE REMOVABLE INSOLE; Not designed for tight fitting dress, casual shoes, or boots; Works best with active shoes such as athletic shoes, cleats, lace up hiking or work boots or most other shoes that have a removable insert. RTP Carbon Fiber Inserts are MADE IN USA. A TEXTURED surface on BOTH sides prevents slipping in shoe, Carbon fiber is used because of its light weight, stiffness, durability, allowing a very thin insert to stiffen the shoe protecting the foot. Not all injuries, pain levels, activity levels, or shoes are the same. 3 levels of stiffness for foot injuries: SEMI-RIGID stiff but flexible, RIGID stiffer but some flex, ARTHRITIC very stiff minimal flex. IF IN DOUBT GO DOWN TO MOST FLEXIBLE INSERT. FOR PROPER FIT CARBON FIBER INSERT MUST not be longer than your foot, measure foot standing; If in between sizes or half sizes round down to smaller size; If necessary, measure in inches and use the ruler in the images to determine the appropriate size in centimeters; to fit the stiff carbon fiber in the shoe, the insert should be ¼ - ½ inch (approximately 1 cm) SHORTER than removable soft insert in shoe. INSERT MUST BE PLACED DIRECTLY UNDER THE REMOVABLE SHOE INSOLE OR UNDER ORTHOTIC; Our inserts were designed by an Athletic Trainer that is a Healthcare provider, engineered by a Mechanical Engineer, Prescribed by sports orthopedic surgeons, sports medicine physicians, podiatrists, chiropractors, athletic trainers, and physical therapists; Used by professional, college, high school, and recreational athletes, as well as active people such as hikers, walkers, and laborers. MAXIMUM STIFFENING INSERT WITH TRACE/MINIMAL FLEXIBILITY Carbon Fiber Insert SOLD AS A SINGLE PIECE, NOT AS A PAIR; HARD, FLAT CARBON FIBER INSERT can be used in EITHER RIGHT OR LEFT SHOE BY SIMPLY TURNING PRODUCT OVER; helps with pain with daily activities with mid-foot arthritis, advanced hallux rigidus post-op care of foot and toe surgeries, or other conditions requiring minimal movement of the foot; Not recommended for athletes to compete in.