by many large German breweries. It slides into place rather than rotating. The M System is very uncommon and is used by only a few breweries in and around Germany (mainly for Aventinus Eisbock, Einbecker, Schneider, Veltins, and Żywiec). It also slides into place. There are two different types of tapping equipment that are available for kegs. A "party tap" or "picnic tap" is a hand-operated pump that utilizes outside air, thus introducing oxygen and bacteria into the keg. This causes the beer to oxidize, affecting the taste; the partial pressure of CO2 will also decrease, causing the beer to go flat. Kegs dispensed with a party pump should be used within 18–24 hours so that the beer does not become unpalatable. Commercial installations, as well as some home users, use pure pressurized gas; these can preserve a keg up to 120 days with proper refrigeration. In simpler installations only CO2 is used to pressurize and dispense the beer, but in installations with very long lines between the keg and dispensing location (bars with customer-operated faucets at each table being an extreme example), the pressure needed to pump the beer for dispensing would over-carbonate the beer. In these situations, "beer gas" or "mixed gas" is used which combines CO2 with another gas, usually nitrogen. Nitrogen is 80 times less soluble in water than CO2, so it can provide additional pressure without noticeably affecting flavor. Typical beer gas is 70-75% nitrogen and 25-30% CO2, but the ideal ratio depends on the beer being served and the installation; more advanced installations blend the gas on site so it can be adjusted for each beer. A few beers like Guinness are required to be pressurized and dispensed with mixed gas; they usually also require the use of a special faucet that deliberately creates additional friction to force the nitrogen out of solution, creating a thick frothy head. As with any pressurized container, a keg can cause injury, even at normal operating pressure, whether with compressed air or carbon dioxide: "The tapping system and pressure regulator both should be equipped with a pressure relief (blow off) device. If you are not familiar with tapping equipment, consult your retailer..." (printed on an Anheuser-Busch's keg cap) Commercially, kegs are usually kept in a refrigerated room for storage and dispensing, and liquid lines run from this room to all of the faucets in the restaurant. Kegs are