Sour (cocktail)

A sour is a traditional family of mixed drinks. "Sours" belong to one of the old families of original cocktails and are described by Jerry Thomas in his 1862 book "How to Mix Drinks". 
Sours are mixed drinks containing a base liquor, lemon or lime juice, and a sweetener (simple syrup or orgeat syrup). Egg whites are also included in some sours.
Description.
Like the daiquiri, the sidecar evolved from the original sour formula, but while sours are characterized by a bright acidity, sidecars are often drier, combining liqueurs like cointreau with citrus. Sidecars are considered more of a challenge for bartenders because the proportion of ingredients is more difficult to balance for liqueurs of variable sweetness.
Types of sours.
Gin sour.
The gin sour is a traditional mixed cocktail that predates prohibition in the United States. It is a simple combination of gin, lemon juice, and sugar. Adding carbonated water to this turns it into a gin fizz.
It was popular during the 1940s, and Kevin Starr includes it in "an array of drinks (the gin sour, the whiskey sour, the Gin Rickey, the Tom Collins, the Pink Lady, the Old Fashioned) that now seem period pieces, evocative of another era."
Pisco sour.
The classic Pisco Sour recipe contains pisco brandy (usually an un-aged grape brandy from Chile and Perú), fresh lime juice, fresh lemon juice, simple syrup, egg white, and bitters. It is shaken, strained, and served straight in a cocktail glass then garnished with the bitters (cinnamon can be used). The addition of egg white creates a foamy head when shaken before serving. While pisco sour is flavoured with key lime by default, pisco is combined with other fruit to create mango sour, maracuya (passion fruit) sour, lucuma sour and so forth. Peru has a National Pisco Sour Day (which lasts a weekend) in mid-February, and Chile has Pisco Day in mid-May.
Rum sour.
The basic Daiquiri, sometimes called a Daquiri sour, is made with rum, lime juice and simple syrup. One account of the drink's origin is that the general manager of the Spanish-American Iron Core Company, which operated the "Daiquiri" mine, ran out of gin one night when making gin sour for his guests, and switched to rum. This rum sour became known as the Daiquiri. However, earlier printed recipes for rum sour exist such as one from Jerry Thomas from 1887.
Whiskey sour.
The whiskey sour is a mixed drink containing bourbon whiskey, lemon juice, sugar, and optionally a dash of egg white to make it a Boston Sour. It is shaken and served either straight or over ice. The traditional garnish is half an orange slice and a maraschino cherry.
A notable variant of the whiskey sour is the Ward 8, which often is based with either bourbon or rye whiskey, both lemon and orange juices, and grenadine syrup as the sweetener. The egg white sometimes employed in other whiskey sours is generally not included in this variation.