A shot glass is an essential measuring tool to use to get uniformity from your espresso machine. An espresso shot should be 1 ounce, and a double-shot should be 2 ounces. The way to calibrate this is to put TWO shotglasses under the double-spout of your portafilter - one under each spout. When you use your espresso machine to extract a double-shot (which is standard in the US), it should fill both shotglasses to the line. The reason that you use two shotglasses is to see that your espresso machine is extracting the shots uniformly. If the tamp is correct and the machine is LEVEL - which it should be - the shots will be the same.
These shot glasses have lines at the 1 ounce mark. This is roughly where your espresso machine should finish the shot. The total capacity of each shot glass is 2 ounces, so there is plenty of room to work with cleanly. You should always callibrate your espresso machine. Using these shotglasses. In Italy, the espresso shot IS 30 milliliters (1 ounce); in America, many set it a little higher (1.5 ounces, max). Either way, these shotglasses will help you to calibrate the shots for your espresso machine daily. An essential item for the professional or serious enthusiast.