Doppio coffee is a double shot of espresso extracted using double the amount of ground coffee in a larger-sized portafilter basket, resulting in 60 ml of drink, double the amount of a single shot espresso. Doppio is Italian for "double" and is commonly called a standard double, due to its standard in judging the espresso quality in barista competitions, where four single espressos are made using two double portafilters. A single shot of espresso is called a solo and was developed because it was the maximum amount of ground coffee that could practically be extracted by lever espresso machines. At most cafes outside of Italy, a doppio is the standard shot. The only difference between doppio and espresso is the serving size. Traditionally, an espresso is 1 oz, and a doppio is 2 oz. A doppio and espresso are made with the same ratio of coffee to water and have the same taste, aroma, and concentration of caffeine. The only difference is in the levels of caffeine. Due to the larger serving size, a doppio espresso contains 140-160mg of caffeine, which is more than 70-80 mg for a single espresso.