Talking with customers about shot sizes and number of shots in a drink can be confusing. Most customers know that shots have something to do with the strength of the coffee but that's about as far as it goes. As baristas we need to understand the details and know when it's appropriate to share them. Often, it's enough to say "all our drinks are made with double shots. We'd be happy to add an extra shot if you'd like it to be stronger." If a customer is interested and has the time invite them away from register to talk coffee.
In Italian espresso tradition, a shot refers to the amount of dry coffee that is used in a basket and the beverage made from that coffee. Every machine came with two different sized baskets for making shots. Single shot baskets held 7 grams and would be used for making a small drink for one person. Double shot baskets held 14 grams and could be used to make a larger shot to be split between two drinks. Over time the double shot became the standard for individual beverages and the basket size grew. Most "double shot" baskets now are 17 or 18 g. Competition baskets are 20 g and triples are 21 g. Basket size is important because it determines how much espresso coffee you can make. The more grinds you use, the more water you can run through those grinds without making the coffee too weak.
Back to Italian tradition, the standard "shot" size was 7 g of grinds to make 14 g of brewed espresso. Or, you can think of it as 2 parts water to 1 part coffee. This 2 to 1 ratio is considered "normale" and is still the standard yield in cafes. Using an 18 g dose this would mean a serving size of 36 g. At the Gryphon, we do a slightly shorter pull yielding 32 g out of an 18 g dose.
Some customers prefer to have their shots pulled "ristretto" or restricted. That would be an 18 g yield for an 18 g dose or 1 to 1 ratio. "Lungo" shots at 2.5 to 1 also have their fans.
It's a great experiment to try espresso at different yields. The flavors change from salty sour becoming smoother and sweeter as more water is added before washing out and turning bland. Check out the Espresso Compass from Barista Hustle to get a good idea of the changes. Or, better yet, pull yourself some shots and let your tongue learn the difference.
Unedited: Any feedback would be appreciated.