By placing two lightweight, high-density ribs at the heel and toe of the iron, TaylorMade has reinforced the perimeter of the club head and increased it's structural rigidity. This in turn allows M4 to localize face flexibility, meaning more energy, ball speed and forgiveness across the hitting area. RIBCOR actively stifles flexibility in the toe, creating a tighter shot dispersion. As an added bonus, RIBCOR makes the M4 irons sound and feel better at impact.