Doppio espresso (italian pronunciation: [ˈdoppjo]) is a double shot which is extracted using double the amount of ground coffee in a larger-sized portafilter basket. this results in 60 ml (2.1 imp fl oz; 2.0 us fl oz) of drink, double the amount of a single shot espresso. doppio is italian multiplier, meaning "double". it is commonly called a standard double, due to its standard in judging the espresso quality in barista competitions, where four single espresso are made using two double portafilters. a single shot of espresso, by contrast, is called a solo ("single") and was developed because it was the maximum amount of ground coffee that could practically be extracted by lever espresso...

Source: Wikipedia