A pressurized strainer, often referred to as a double-walled strainer, is a special type of strainer (bowl) inserted into the lever of home coffee makers. Its main purpose is to artificially create the pressure needed to extract the espresso. Unlike standard strainers with many holes, a pressure relief strainer has only one or a few small holes on the outside. Inside there is a second wall with more holes. Thus, water first flows through the ground coffee and is then forced under pressure through the small outlet hole. This mechanism helps to create the foam known as "crema" even if the coffee is not ground to the ideal coarseness for espresso or if pre-ground coffee is used. It is suitable for beginners or for coffee machines that are not equipped with a quality grinder.