Negroni Week arrives on the 21st of September, and for one glorious fortnight the whole drinks world turns the colour of a London bus. Before you reach for the gin, though, consider the drink that came first. The Americano is the Negroni's older, gentler ancestor: Campari and sweet vermouth lengthened with soda, all bitter orange and fizz, and low enough in strength that you can have two before dinner and still find dinner. It is the great long aperitivo, and early autumn is exactly its moment.

The story runs backwards from the drink most people know. Count Camillo Negroni, so the legend goes, walked into a Florence bar around 1919 and asked for his usual Americano with the soda swapped for gin. The barman obliged, the Negroni was born, and the original quietly slipped into the background. Which is a shame, because the Americano is a lovely thing in its own right, and not merely a Negroni with the training wheels still on.

It began life in Milan under a less romantic name. Mix Campari, which is made in Milan, with sweet vermouth from Turin, and you have a Milano-Torino, a bittersweet pairing poured since the 1860s. Add a splash of soda and the drink picked up a new name in the early twentieth century, reputedly in honour of the American drinkers who took to it in Gaspare Campari's bar. Americano it has been ever since.

It has a decent claim to literary pedigree, too. The very first drink James Bond orders in print is an Americano, in the opening chapters of Ian Fleming's Casino Royale, long before the Martini became his signature. Bond, it turns out, understood the assignment: an Americano is what you drink when the evening is still young and you intend to keep your wits about you.

The formula

Learn the shape and you will never need a recipe. Equal parts Campari and sweet vermouth over plenty of ice, then top with soda water to taste. Twenty-five millilitres of each is the standard pour, lengthened with around twice as much soda, though the beauty of the drink is that it forgives a heavy or a light hand. Build it straight in the glass, give it the gentlest stir, and finish with a slice of orange. No shaker, no straining, no ceremony.

The glass is part of it. An Americano wants length, so reach for a highball or a large tumbler rather than the squat rocks glass a Negroni lives in. You are making a drink to sip slowly while the light goes, not a short, stiff one to see off in three mouthfuls. Get the proportion of soda right and it should taste bright and bitter and faintly sweet, with the fizz carrying the whole thing along.

The Campari

Campari is the drink's beating heart, and its whole personality. That vivid, unmistakable red is bittersweet and gently medicinal in the best possible sense, all bitter orange, rhubarb and a long, dry finish. Campari Original was first mixed by Gaspare Campari behind his Milan bar in 1860, and the recipe has stayed a closely guarded secret ever since. In an Americano it has more room to breathe than it does in a Negroni, softened by the soda rather than braced by gin, so its bitter-orange character really sings.

The vermouth

The other half of the drink is sweet red vermouth, and it is worth taking as seriously as the Campari. Carpano Antica Formula is the benchmark: dark, spiced, almost chocolatey, with a vanilla depth that rounds off Campari's sharp corners and gives the drink real weight. Antonio Benedetto Carpano more or less invented vermouth in Turin in 1786, which is a decent head start on the competition. If you would rather something lighter and more floral, Dolin Rouge from Chambery makes a prettier, more delicate Americano that leans into the drink's softer side.

One rule matters more than any other: vermouth is fortified wine, and it goes off. Keep the bottle in the fridge once it is open and use it while it is fresh, within a few weeks rather than a few months. A tired, oxidised vermouth is the single most common reason a home Americano tastes flat and joyless, and it is also the easiest fault in the drink to put right.

The soda, the ice and the orange

With only three parts, the details are the drink. Use soda from a fresh bottle or a good syphon, because flat soda makes a flat Americano. Fill the glass properly with ice; a generous amount actually keeps the drink colder and melts more slowly than a mean scattering of cubes. Then the orange. A half-wheel dropped in looks handsome, but for the best of it, pare a strip of peel, twist it over the surface so the oils catch the fizz, and run it round the rim. Some bars finish with lemon for a sharper, brighter note, and that is a fine choice on a warmer evening.

When to pour one

The Americano is an aperitivo in the truest sense, which is to say a drink with a job: to wake the appetite rather than flatten it. Pour one an hour before dinner, put out a small bowl of good olives, some salted almonds or a few crisps, and let the bitterness do its work. Its lower strength is the secret weapon. Where a second Negroni can quietly end an evening, a second Americano simply extends a good one, which makes it the ideal drink for the slow, sociable stretch between finishing work and thinking about food.

Milano-Torino, and where to go next

Once the Americano is second nature, a whole family opens up. Leave out the soda and you are back at the Milano-Torino, a short, intense sip that is all bitter depth and no fizz. Swap the soda for a splash of gin and you have, of course, the Negroni itself: the same DNA, turned up to full volume. Replace the gin with prosecco instead and you drift toward a Negroni Sbagliato, the mistaken Negroni that had its viral moment a few years back and never quite left. The Americano sits at the gentle end of all of them, which is precisely the point.

So yes, celebrate Negroni Week when it comes. But there is a strong case for starting the fortnight one step earlier, with the drink Count Negroni was holding before he asked for gin. Pour equal parts, lengthen with soda, drop in the orange and take it somewhere you can sit for a while. Made properly, an Americano is not a drink you rush. It is the long, civilised opening act of the evening, and there is no better time of year to get to know it.