In the classic method the base wine is then bottled and will undergo a second fermentation in the bottle. In this stage (known as 'tirage') sugar is added to the base wine, along with selected yeasts and riddling agents. This mixture is known as 'liqueur de tirage'. Once they have been filled the bottles are closed with a special stopper, a cylinder placed with the opening facing down towards the wine (the 'bidule' or cartridge), over which the better-known crown cork is applied.