Despite often having a slightly tainted reputation in some modern bars for being over sweet or sickly, sours do have an extremely rich history within the cocktail world. They were first described, in print, by Jerry Thomas in 1862 within his book How To Mix Drinks. A sour is described as a mixed drink containing a base liquor, lemon / lime juice, egg white and a sweetener. You scan broadly say a sour should be 2 part liquor, 1 part souring agent, 1/2 part sweetener.
There are many variations including Margaritas and Sidecars which include a second liquor such as cointreau or triple sec. The White Lady (aka Chelsea Sidecar, Delilah and Lillian Forever) can be equated with a sidecar but with the brandy substituted for gin.
It's origin is, as always, under dispute. One story come from Harry MacElhone, a bartender at Ciro's Club in 1919. His original recipe called for creme de menthe, which was later substituted for gin when he worked in Harry's New York Bar in 1929.
A slightly more popular version comes from Harry Cradock, of The Savoy. The recipe appears n his book Savoy Cocktail Book published in 1930. Further evidence came from a Channel 4 documentary where Joe Gilmore, who was also a head bartender at The Savoy, talks about how the drink was a favourite of Laurel and Hardy.
The recipe that appears in The Savoy Cocktail book calls for 1/2 dry gin, 1/4 cointreau, and 1/4 lemon. This results in a fairly tart, dry drink that would be closer to a martini in style rather than a typical sour including egg white etc. The drink still acts as an excellent aperitif
The recipe has not undergone much transformation in recent times. In the US during the 1940's it enjoyed a fleeting relationship with egg-white, as reported by Oscar Haimo in 1943 and subsequently reached the UK shores in 1954 in a book called Shake Again with Eddie where egg-white is included as a side note. A variation also exists called a Perfect Lady which substitutes peach brandy for cointreau. This version took first prize in the Empire Cocktail Competition, 1936.