1 part Vermouth
1 part Gin/Vodka
As with many cocktails the Martini’s beginnings is up to legend an overview of the story is this:
Originates from a number of different drinks, one of which the Manhattan. To start with, in the 1880s, the Martini was made with gin and vermouth. Towards the end of the decade consumers demanded drier drinks, that with dry gins coming to the market the Martininez became popular. The recipe for this is bitters, maraschino, gin and vermouth. By 1900 the Dry Martini was created using dry gin and vermouth which became more popular than the others.
The specifics are up for debate.
One story is in Martinez, California a miner walked into a bar and the bartender made him a ‘Martinez’ which was vermouth, gin, syrup and maraschino. The miner proceeded to drink so many he drop the ‘z’ and so the Martini was born. In Food 52 they continue by saying that it is in fact the miner was on his way to Martinez and requested a pick me up.
Another is that it was created in New York City in Knickerbocker Hotel, one of the bartenders Martini di Arma di Taggia, who served a drink to John D. Rockerfeller that used gin, vermouth and orange bitters and thus was named after himself. Another version of the story is still set in the hotel but the drink was named after the vermouth Martini.
The recipe is varied as much as the origins are. Originally the ration was 1:1 now it is down to taste. There are variations like the elderflower martini which is vodka, elderflower liqueur and martini (4:2:1). Or the Espresso Martini which is vodka, espresso coffee, coffee liqueur and sugar syrup. (6:6:3:1)