Adding two dashes of Angostura bitters to a gin sour makes it a Fitzgerald, which also has 50 ml of gin, 25 ml of lemon juice, and 15 ml of sugar syrup.
Adding carbonated water (soda water) to a gin sour makes it a gin fizz. Add egg white (to a gin fizz) to make a Silver Fizz, egg yolk to make a Golden Fizz, a whole egg to make a Royal Fizz, or crème de menthe to make a Green Fizz. Use sparkling wine instead of water to make a Diamond Fizz (a.k.a. French 75).
The gin fizz is similar to a Tom Collins, but that was originally specified to use Old Tom gin – a type of gin that's sweeter than the Dry London variety but not as sweet as Dutch or Holland gin (jenever). If made with jenever, it's a John Collins. Invented in 1876, the Tom Collins can be made with or without carbonated water (depending on the recipe). It went out of fashion in the 1920s.
© Haydn Thompson 2017