The Sour cocktail is the great grandfather of many modern cocktails. The Cosmopolitan comes to mind as one of these modern grand-children. The Sour has begot many, many variations along the way. So this means that once a bartender has mastered it they have the basic knowledge needed for hundreds of other drinks.
The Sour cocktail is a balanced mix of sweet and tart components that have a complex nature to it. Add something like whiskey and then you really have something. Finding the sweet spot, pun intended, with the right ratio of liquor, sweet, and tart, takes trial and error. The other complexity is that this ratio will vary based on the liquor used.
The base liquor used in a Sour can be any of the 8 staples: bourbon, brandy, gin, rum, scotch, tequila, vodka, or whiskey. So the drink is called... Bourbon Sour, Brandy Sour, Gin Sour, …, and Whiskey Sour.
Sours should be served in a goblet shaped glass called a sour glass, but an old-fashioned glass will do in a pinch.
The Sour is one of those cocktails that work best in the home bar. The reason for this is at home you can adjust your sweet and sour levels to fit your personal tastes. In a bar environment the sour has to be generalized and it doesn't quite meet everyone's pleasure. That is unless you have a very inquisitive bar chef serving it up.
This is a generalized mix that you can start your experimentation with:
2 parts Base Liquor
1 part Simple Syrup
.75 part Lemon Juice
The Sour is not only the name of a cocktail it is also a whole category of cocktails. The cocktail family called Sours contains a mixture of base liquor, lemon or lime juice, and a non-alcoholic sweetening agent. Some examples of the sweetening agents include simple syrup, grenadine, or pineapple juice. If the base liquor is a liqueur then the sweetening agent may not be needed. (1)
The Sour family was first described by "The Professor" Jerry Thomas in his 1862 book, How to Mix Drinks. Since that time the Sour's basic recipe has been built on to develop many of today's most popular cocktails.
There is a sub-classification, as defined by Gary Regan, of the Sour called the New Orleans Sours which are those Sours that are sweetened by orange liqueur (Triple Sec, Cointreau, Curacao, etc...). I mention this sub-category as it has some very important standard cocktails in it, to name a few: Cosmopolitan, Margarita, Metropolitan, and Sidecar. (1)
Because there are now so many cocktails based on this sweet and sour mixture there are a few problems with this very important category in bar rooms today...
If you go into many bars right now you will get a sad resemblance of what a true Sour cocktail is supposed to be. There are many commercially available sweet and sour mixes that is now being used as a shortcut to this large category of cocktails. This cocktail is much, much better with fresh ingredients.
The mix also does not allow the bartender to vary the sweet and sour ratio. This can be an issue because the same mix that is used in a Frozen Margarita is also used in a Whiskey Sour. The Frozen Margarita should be a lot sweeter and the Whiskey Sour should be more on the sour side. Commercial mixes aside from not tasting as good actually cause more of a headache when you want the right proportions.
Another perversion of the Sour comes with the addition of egg white to the cocktail. This is done in order to get a frothy head on the drink. This exact same frothy head on the Sour, which is supposed to be there, can be achieved with the proper amount of vigorous shaking. The Sour should be shook very hard for a slow ten count. When it is poured back into its glass, there will be foam... without egg white.
To read more about this very important old stand-by I highly recommend Gary Ragan’s The Joy of Mixology.