A wine bottle is a bottle used for holding wine, generally made of glass. Some wines are fermented in the bottle, others are bottled only after fermentation. Recently, the bottle has become a standard unit of volume to describe sales in the wine industry, measuring 750 millilitres (26 imp fl oz; 25 US fl oz).[citation needed] However, bottles are produced in a variety of volumes and shapes. Wine bottles are traditionally sealed with cork, but screw-top caps are becoming popular, and there are several other methods used to seal a bottle.