After tirage the wine is immediately bottled in extremely thick glass and securely sealed. The bottles are stored horizontally in specially designed stands with the temperature maintained at about 55º F . The alcoholic fermentation begins almost immediately, increasing the alcohol content by 1-2%, and the carbon dioxide bi-product is trapped and therefore infuses into the wine. Slowly, the yeast cells die off and collect at the lowest point in the bottle as the added sugars are converted into alcohol. These dead yeast cells are called lees, and are responsible for much of the champagne's flavor development acquired during the "ageing on lees". This period can last from 1-4 years depending on the intended quality and style of the champagne.