The Pin Index Safety System, or PISS, is a safety system that uses geometric features on the yoke to ensure that pneumatic connections between a gas cylinder and a machine that uses pressurized gases are not connected to the wrong gas yoke. This system can be seen on an anesthesia machine.

Each gas cylinder has a pin configuration to fit its respective gas yoke. Refer to the figure on the right; units are in millimeters.

• O₂
  Oxygen
  Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
  
  : 2-5
• N₂O
  Nitrous oxide
  Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
  
  : 3-5
• Air
  Earth's atmosphere
  The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
  
  : 1-5
• CO₂
  Carbon dioxide
  Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
  
  : 1-6
• Heliox
  Heliox
  Heliox is a breathing gas composed of a mixture of helium and oxygen .Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of...
  
  : 2-4

Entonox= 7

Standards

• International (ISO
  International Organization for Standardization
  The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
  
  ) and European (CEN
  CEN
  CEN may refer to:*Cen, or sen, is the currency of the fictional nation of Amestris of the anime/manga "Fullmetal Alchemist"As a three-letter acronym:* Cambridge Evening News, former name for the Cambridge News...
  
  ) Standard
• EN ISO 407 : Small medical gas cylinders - Pin-index yoke-type valve connections

The pin index system is a safety system (PISS) designed to ensure the correct gas is filled into the correct cylinder, and that the cylinder will only connect to the correct equipment. The positions of the holes on the cylinder valve correspond with the pins fitted to the yoke attached to the equipment. The pin positions for each medical gas are unique. If an attempt is made to fit the wrong gas cylinder to the yoke a tight seal will not be made, as the pins cannot locate.