IEC 60529 outlines an international classification system for the sealing effectiveness of enclosures utilizing the letters "IP” followed by two digits which represents ingress of solid objects and liquids respectively. Ratings typically are two letters and two digits such as “IP54” or “IP65”. However, a third digit is sometimes used to represent protection against mechanical impact. Also, an X may also be used in lieu of one of the digits if there is only one class of protection, such as IPX5 which only addresses moisture resistance.