The martini is a cocktail made with gin and vermouth, and garnished with an olive or a lemon twist. Over the years, the martini has become one of the best-known mixed alcoholic beverages.

By 1922 the martini reached its most recognizable form in which London dry gin and dry vermouth are combined at a ratio of 2:1, stirred in a mixing glass with ice cubes, with the optional addition of orange or aromatic bitters, then strained into a chilled cocktail glass.[3] Over time the generally expected garnish became the drinker’s choice of a green olive or a twist of lemon peel.

A dry martini is made with little to no vermouth. Ordering a martini “extra dry” will result in even less or no vermouth added. By the Roaring Twenties, it became a common drink order. Over the course of the 20th century, the amount of vermouth steadily dropped. During the 1930s the ratio was 3:1 (gin to vermouth), and during the 1940s the ratio was 4:1. During the latter part of the 20th century, 5:1 or 6:1 dry martinis became considered the norm. Drier variations can go to 8:1, 12:1, 15:1.

From Wikipedia