 Although theoretically trimix can be blended with almost any combination of helium and oxygen, a number of "standard" mixes have evolved (such as 21/35, 18/45 and 15/55—see "Naming conventions").Most of these mixes originated from filling the cylinders with a certain percentage of helium, and then topping the mix with 32% enriched air nitrox.The "standard" mixes evolved because of three coinciding factors—the desire to keep that equivalent narcotic depth (END) of the mix at approximately , the requirement to keep the partial pressure of oxygen at 1.4 ATA or below at the deepest point of the dive, and the fact that many dive shops stored standard 32% enriched air nitrox in banks, which simplified mixing.